Re: [PATCH 3/3] Reenter guest after instruction emulation failure if emulation was due to access to non-mmio address.

[Avi Kivity <avi@redhat.com>]

On 07/07/2010 08:16 PM, Gleb Natapov wrote:
> When shadow pages are in use sometimes KVM try to emulate an instruction
> when it accesses a shadowed page. If emulation fails KVM un-shadows the
> page and reenter guest to allow vcpu to execute the instruction. If page
> is not in shadow page hash KVM assumes that this was attempt to do MMIO
> and reports emulation failure to userspace since there is no way to fix
> the situation. This logic has a race though. If two vcpus tries to write
> to the same shadowed page simultaneously both will enter emulator, but
> only one of them will find the page in shadow page hash since the one who
> founds it also removes it from there, so another cpu will report failure
> to userspace and will abort the guest.
>
> Fix this by checking (in addition to checking shadowed page hash) that
> page that caused the emulation belongs to valid memory slot. If it is
> then reenter the guest to allow vcpu to reexecute the instruction.
>
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7070b41..dd7b241 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4000,6 +4000,8 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
>   		if (r)  {
>   			if (kvm_mmu_unprotect_page_virt(vcpu, cr2))
>   				return EMULATE_DONE;
> +			if (!kvm_is_error_hva(gfn_to_hva(vcpu->kvm, cr2)))
> +				return EMULATE_DONE;
>    

cr2 is a gva, not a gfn.

cr2 is not always valid, but I guess that's for a later patch.

>   			if (emulation_type&  EMULTYPE_SKIP)
>   				return EMULATE_FAIL;
>   			return handle_emulation_failure(vcpu);
> @@ -4026,6 +4028,8 @@ restart:
>   		 */
>   		if (kvm_mmu_unprotect_page_virt(vcpu, cr2))
>   			return EMULATE_DONE;
> +		if (!kvm_is_error_hva(gfn_to_hva(vcpu->kvm, cr2)))
> +			return EMULATE_DONE;
>
>    

Code is duplicated.  Helper?

>   		return handle_emulation_failure(vcpu);
>   	}
>    


-- 
error compiling committee.c: too many arguments to function