[2/2] KVM: x86: Set EMULTYPE_NO_REEXECUTE for MMIO emulation
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Message ID 20180822215225.28703-3-sean.j.christopherson@intel.com
State New
Headers show
Series
  • KVM: x86: Don't re-{try,execute} L2 instrs
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Commit Message

Sean Christopherson Aug. 22, 2018, 9:52 p.m. UTC
reexecute_instruction(), added by commit a6f177efaa58 ("KVM: Reenter
guest after emulation failure if due to access to non-mmio address"),
is intended to return true if and only if emulation failed due to a
write to a shadowed page, i.e. MMIO emulation that happens to fail
should never be retried.  Set EMULTYPE_NO_REEXECUTE when we know we
are emulating a (alleged) MMIO access to document that we should not
re-execute the instruction.

This is purely a documentation update as reexecute_instruction() does
not re-execute instructions whose faulting GPA corresponds to an error
or no-slot PFN, e.g. EMULTYPE_NO_REEXECUTE was added by commit
991eebf9f8e5 ("KVM: VMX: do not try to reexecute failed instruction
while emulating invalid guest state") for reasons entirely unrelated
to MMIO.  Case in point, mmio_info_in_cache() always returns false for
nested VMs, e.g. EMULTYPE_NO_REEXECUTE won't be set if MMIO emulation
for a nested VM is triggered outside of the handle_mmio_page_fault()
path.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kvm/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Paolo Bonzini Aug. 23, 2018, 9:07 a.m. UTC | #1
On 22/08/2018 23:52, Sean Christopherson wrote:
> reexecute_instruction(), added by commit a6f177efaa58 ("KVM: Reenter
> guest after emulation failure if due to access to non-mmio address"),
> is intended to return true if and only if emulation failed due to a
> write to a shadowed page, i.e. MMIO emulation that happens to fail
> should never be retried.  Set EMULTYPE_NO_REEXECUTE when we know we
> are emulating a (alleged) MMIO access to document that we should not
> re-execute the instruction.
> 
> This is purely a documentation update as reexecute_instruction() does
> not re-execute instructions whose faulting GPA corresponds to an error
> or no-slot PFN, e.g. EMULTYPE_NO_REEXECUTE was added by commit
> 991eebf9f8e5 ("KVM: VMX: do not try to reexecute failed instruction
> while emulating invalid guest state") for reasons entirely unrelated
> to MMIO.  Case in point, mmio_info_in_cache() always returns false for
> nested VMs, e.g. EMULTYPE_NO_REEXECUTE won't be set if MMIO emulation
> for a nested VM is triggered outside of the handle_mmio_page_fault()
> path.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/x86/kvm/mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index a282321329b5..1c152fd9fc7c 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -5231,7 +5231,7 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
>  	if (unlikely(error_code & PFERR_RSVD_MASK)) {
>  		r = handle_mmio_page_fault(vcpu, cr2, direct);
>  		if (r == RET_PF_EMULATE) {
> -			emulation_type = 0;
> +			emulation_type = EMULTYPE_NO_REEXECUTE;
>  			goto emulate;
>  		}
>  	}
> @@ -5261,7 +5261,7 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
>  	}
>  
>  	if (mmio_info_in_cache(vcpu, cr2, direct))
> -		emulation_type = 0;
> +		emulation_type = EMULTYPE_NO_REEXECUTE;
>  emulate:
>  	/*
>  	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
> 

Even better, EMULTYPE_NO_REEXECUTE is never set if EMULTYPE_RETRY is
set, and vice versa.  The sole exception is rsm_interception in svm.c
that should actually specify EMULTYPE_NO_REEXECUTE but doesn't.

So it seems to me that they can be unified to a single EMULTYPE_RETRY,
which makes sense since they do basically the same thing---only
EMULTYPE_NO_REEXECUTE is a bit more desperado because decoding has failed.

Paolo

Patch
diff mbox series

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index a282321329b5..1c152fd9fc7c 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5231,7 +5231,7 @@  int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 	if (unlikely(error_code & PFERR_RSVD_MASK)) {
 		r = handle_mmio_page_fault(vcpu, cr2, direct);
 		if (r == RET_PF_EMULATE) {
-			emulation_type = 0;
+			emulation_type = EMULTYPE_NO_REEXECUTE;
 			goto emulate;
 		}
 	}
@@ -5261,7 +5261,7 @@  int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 	}
 
 	if (mmio_info_in_cache(vcpu, cr2, direct))
-		emulation_type = 0;
+		emulation_type = EMULTYPE_NO_REEXECUTE;
 emulate:
 	/*
 	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.