Re: [PATCH 2/2] KVM: Fix writeback on page boundary that propagate changes in spite of #PF

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jan 11, 2012 at 06:53:31PM +0200, Nadav Amit wrote:
> Consider the case in which an instruction emulation writeback is performed on a page boundary.
> In such case, if a #PF occurs on the second page, the write to the first page already occurred and cannot be retracted.
> Therefore, validation of the second page access must be performed prior to writeback.
> 
> Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
> ---
>  arch/x86/kvm/x86.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 05fd3d7..7af3d67 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3626,6 +3626,8 @@ struct read_write_emulator_ops {
>  			       int bytes, void *val);
>  	int (*read_write_exit_mmio)(struct kvm_vcpu *vcpu, gpa_t gpa,
>  				    void *val, int bytes);
> +	gpa_t (*read_write_validate)(struct kvm_vcpu *vcpu, gva_t gva,
> +				     struct x86_exception *exception);
>  	bool write;
>  };
>  
> @@ -3686,6 +3688,7 @@ static struct read_write_emulator_ops write_emultor = {
>  	.read_write_emulate = write_emulate,
>  	.read_write_mmio = write_mmio,
>  	.read_write_exit_mmio = write_exit_mmio,
> +	.read_write_validate = kvm_mmu_gva_to_gpa_write,
>  	.write = true,
>  };
>  
> @@ -3750,6 +3753,16 @@ int emulator_read_write(struct x86_emulate_ctxt *ctxt, unsigned long addr,
>  		int rc, now;
>  
>  		now = -addr & ~PAGE_MASK;
> +
> +		/* First check there is no page-fault on the next page */
> +		if (ops->read_write_validate &&
> +			ops->read_write_validate(vcpu, addr+now, exception) ==
> +			UNMAPPED_GVA) {
> +			/* #PF on the first page should be reported first */
> +			ops->read_write_validate(vcpu, addr, exception);
> +			return X86EMUL_PROPAGATE_FAULT;
> +		}
> +
This undoes optimization that vcpu_mmio_gva_to_gpa() has for handling
mmio. Furthermore for common (non faulting) case we will check page
tables twice on each write that crosses page boundary, first time here
and second time in emulator_read_write_onepage().

>  		rc = emulator_read_write_onepage(addr, val, now, exception,
>  						 vcpu, ops);
>  
> -- 
> 1.7.4.1
> 
> --
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--
			Gleb.