/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* Asm versions of Xen pv-ops, suitable for direct use.
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*
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* We only bother with direct forms (ie, vcpu in pda) of the
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* operations here; the indirect forms are better handled in C.
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*/
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#include <asm/thread_info.h>
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#include <asm/processor-flags.h>
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#include <asm/segment.h>
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#include <asm/asm.h>
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#include <xen/interface/xen.h>
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#include <linux/linkage.h>
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/* Pseudo-flag used for virtual NMI, which we don't implement yet */
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#define XEN_EFLAGS_NMI 0x80000000
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/*
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* This is run where a normal iret would be run, with the same stack setup:
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* 8: eflags
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* 4: cs
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* esp-> 0: eip
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*
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* This attempts to make sure that any pending events are dealt with
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* on return to usermode, but there is a small window in which an
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* event can happen just before entering usermode. If the nested
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* interrupt ends up setting one of the TIF_WORK_MASK pending work
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* flags, they will not be tested again before returning to
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* usermode. This means that a process can end up with pending work,
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* which will be unprocessed until the process enters and leaves the
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* kernel again, which could be an unbounded amount of time. This
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* means that a pending signal or reschedule event could be
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* indefinitely delayed.
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*
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* The fix is to notice a nested interrupt in the critical window, and
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* if one occurs, then fold the nested interrupt into the current
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* interrupt stack frame, and re-process it iteratively rather than
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* recursively. This means that it will exit via the normal path, and
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* all pending work will be dealt with appropriately.
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*
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* Because the nested interrupt handler needs to deal with the current
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* stack state in whatever form its in, we keep things simple by only
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* using a single register which is pushed/popped on the stack.
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*/
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.macro POP_FS
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1:
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popw %fs
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.pushsection .fixup, "ax"
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2: movw $0, (%esp)
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jmp 1b
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.popsection
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_ASM_EXTABLE(1b,2b)
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.endm
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ENTRY(xen_iret)
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/* test eflags for special cases */
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testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
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jnz hyper_iret
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push %eax
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ESP_OFFSET=4 # bytes pushed onto stack
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/* Store vcpu_info pointer for easy access */
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#ifdef CONFIG_SMP
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pushw %fs
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movl $(__KERNEL_PERCPU), %eax
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movl %eax, %fs
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movl %fs:xen_vcpu, %eax
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POP_FS
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#else
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movl %ss:xen_vcpu, %eax
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#endif
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/* check IF state we're restoring */
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testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp)
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/*
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* Maybe enable events. Once this happens we could get a
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* recursive event, so the critical region starts immediately
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* afterwards. However, if that happens we don't end up
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* resuming the code, so we don't have to be worried about
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* being preempted to another CPU.
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*/
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setz %ss:XEN_vcpu_info_mask(%eax)
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xen_iret_start_crit:
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/* check for unmasked and pending */
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cmpw $0x0001, %ss:XEN_vcpu_info_pending(%eax)
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/*
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* If there's something pending, mask events again so we can
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* jump back into xen_hypervisor_callback. Otherwise do not
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* touch XEN_vcpu_info_mask.
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*/
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jne 1f
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movb $1, %ss:XEN_vcpu_info_mask(%eax)
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1: popl %eax
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/*
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* From this point on the registers are restored and the stack
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* updated, so we don't need to worry about it if we're
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* preempted
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*/
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iret_restore_end:
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/*
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* Jump to hypervisor_callback after fixing up the stack.
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* Events are masked, so jumping out of the critical region is
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* OK.
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*/
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je xen_hypervisor_callback
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1: iret
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xen_iret_end_crit:
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_ASM_EXTABLE(1b, iret_exc)
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hyper_iret:
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/* put this out of line since its very rarely used */
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jmp hypercall_page + __HYPERVISOR_iret * 32
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.globl xen_iret_start_crit, xen_iret_end_crit
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/*
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* This is called by xen_hypervisor_callback in entry.S when it sees
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* that the EIP at the time of interrupt was between
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* xen_iret_start_crit and xen_iret_end_crit. We're passed the EIP in
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* %eax so we can do a more refined determination of what to do.
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*
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* The stack format at this point is:
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* ----------------
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* ss : (ss/esp may be present if we came from usermode)
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* esp :
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* eflags } outer exception info
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* cs }
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* eip }
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* ---------------- <- edi (copy dest)
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* eax : outer eax if it hasn't been restored
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* ----------------
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* eflags } nested exception info
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* cs } (no ss/esp because we're nested
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* eip } from the same ring)
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* orig_eax }<- esi (copy src)
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* - - - - - - - -
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* fs }
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* es }
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* ds } SAVE_ALL state
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* eax }
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* : :
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* ebx }<- esp
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* ----------------
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*
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* In order to deliver the nested exception properly, we need to shift
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* everything from the return addr up to the error code so it sits
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* just under the outer exception info. This means that when we
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* handle the exception, we do it in the context of the outer
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* exception rather than starting a new one.
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*
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* The only caveat is that if the outer eax hasn't been restored yet
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* (ie, it's still on stack), we need to insert its value into the
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* SAVE_ALL state before going on, since it's usermode state which we
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* eventually need to restore.
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*/
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ENTRY(xen_iret_crit_fixup)
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/*
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* Paranoia: Make sure we're really coming from kernel space.
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* One could imagine a case where userspace jumps into the
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* critical range address, but just before the CPU delivers a
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* GP, it decides to deliver an interrupt instead. Unlikely?
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* Definitely. Easy to avoid? Yes. The Intel documents
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* explicitly say that the reported EIP for a bad jump is the
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* jump instruction itself, not the destination, but some
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* virtual environments get this wrong.
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*/
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movl PT_CS(%esp), %ecx
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andl $SEGMENT_RPL_MASK, %ecx
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cmpl $USER_RPL, %ecx
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je 2f
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lea PT_ORIG_EAX(%esp), %esi
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lea PT_EFLAGS(%esp), %edi
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/*
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* If eip is before iret_restore_end then stack
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* hasn't been restored yet.
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*/
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cmp $iret_restore_end, %eax
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jae 1f
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movl 0+4(%edi), %eax /* copy EAX (just above top of frame) */
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movl %eax, PT_EAX(%esp)
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lea ESP_OFFSET(%edi), %edi /* move dest up over saved regs */
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/* set up the copy */
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1: std
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mov $PT_EIP / 4, %ecx /* saved regs up to orig_eax */
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rep movsl
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cld
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lea 4(%edi), %esp /* point esp to new frame */
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2: jmp xen_do_upcall
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