Re: [PATCH 03/11] KVM: x86: retry non-page-table writing instruction

Marcelo Tosatti
2011-08-22T20:02:11Z

On Tue, Aug 16, 2011 at 02:42:07PM +0800, Xiao Guangrong wrote:
> If the emulation is caused by #PF and it is non-page_table writing instruction,
> it means the VM-EXIT is caused by shadow page protected, we can zap the shadow
> page and retry this instruction directly
>
> The idea is from Avi
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> ---
> arch/x86/include/asm/kvm_emulate.h | 1 +
> arch/x86/include/asm/kvm_host.h | 5 +++
> arch/x86/kvm/emulate.c | 5 +++
> arch/x86/kvm/mmu.c | 22 +++++++++++---
> arch/x86/kvm/x86.c | 53 ++++++++++++++++++++++++++++++++++++
> 5 files changed, 81 insertions(+), 5 deletions(-)
>
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4814,6 +4814,56 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
> return false;
> }
>
> +static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
> + unsigned long cr2, int emulation_type)
> +{
> + struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> + unsigned long last_retry_eip, last_retry_addr, gpa = cr2;
> +
> + last_retry_eip = vcpu->arch.last_retry_eip;
> + last_retry_addr = vcpu->arch.last_retry_addr;
> +
> + /*
> + * If the emulation is caused by #PF and it is non-page_table
> + * writing instruction, it means the VM-EXIT is caused by shadow
> + * page protected, we can zap the shadow page and retry this
> + * instruction directly.
> + *
> + * Note: if the guest uses a non-page-table modifying instruction
> + * on the PDE that points to the instruction, then we will unmap
> + * the instruction and go to an infinite loop. So, we cache the
> + * last retried eip and the last fault address, if we meet the eip
> + * and the address again, we can break out of the potential infinite
> + * loop.
> + */
> + vcpu->arch.last_retry_eip = vcpu->arch.last_retry_addr = 0;
> +
> + if (!(emulation_type & EMULTYPE_RETRY))
> + return false;
> +
> + if (page_table_writing_insn(ctxt))
> + return false;
> +
> + if (ctxt->eip == last_retry_eip && last_retry_addr == cr2)
> + return false;
> +
> + vcpu->arch.last_retry_eip = ctxt->eip;
> + vcpu->arch.last_retry_addr = cr2;
> +
> + if (!vcpu->arch.mmu.direct_map && !mmu_is_nested(vcpu))
> + gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2, NULL);

Why write?

> + kvm_mmu_unprotect_page(vcpu->kvm, gpa >> PAGE_SHIFT);
> +
> + /*
> + * The shadow pages have been zapped, then we call the page
> + * fault path to change the mapping to writable.
> + */
> + vcpu->arch.mmu.page_fault(vcpu, cr2, PFERR_WRITE_MASK, true);

I don't see why is this necessary. Just allowing the instruction to
proceed should be enough?

Looks good otherwise.