linux-kernel - Re: [PATCH v2] KVM: x86: Avoid guest page table walk when gpa

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Message-ID: <20170530090548.GA2764@8bytes.org>
Date:   Tue, 30 May 2017 11:05:48 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, Thomas.Lendacky@....com
Subject: Re: [PATCH v2] KVM: x86: Avoid guest page table walk when
 gpa_available is set

On Fri, May 19, 2017 at 11:12:29AM -0400, Brijesh Singh wrote:
> From: Brijesh Singh <brijesh.singh@....com>
> 
> On AMD hardware when a guest causes a NPF which requires emulation,
> the vcpu->arch.gpa_available flag is set to indicate that cr2 contains
> a valid GPA.
> 
> Currently, emulator_read_write_onepage() makes use of gpa_available flag
> to avoid a guest page walk for a known MMIO regions. Lets not limit
> the gpa_available optimization to just MMIO region. The patch extends
> the check to avoid page walk whenever gpa_available flag is set.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
> v1: http://marc.info/?l=kvm&m=149304930814202&w=2
> 
> Changes in v2:
>  - move gpa_val setting in pf_interception
> 
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/svm.c              |  4 ++++
>  arch/x86/kvm/x86.c              | 14 +++++++-------
>  3 files changed, 12 insertions(+), 7 deletions(-)

I havn't checked in detail, but maybe you have: Does that take nesting
into account, where we might run on a shadow nested page-table and we
actually get a nested gpa (which still needs translation) instead of a
normal gpa?


	Joerg