linux-kernel - Re: [RFC PATCH v1 01/28] kvm: svm: Add support for additional SVM NPF error codes

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Message-ID: <20160913095604.GB19076@nazgul.tnic>
Date:   Tue, 13 Sep 2016 11:56:04 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     simon.guinot@...uanux.org, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, rkrcmar@...hat.com, matt@...eblueprint.co.uk,
        linus.walleij@...aro.org, linux-mm@...ck.org,
        paul.gortmaker@...driver.com, hpa@...or.com,
        dan.j.williams@...el.com, aarcange@...hat.com,
        sfr@...b.auug.org.au, andriy.shevchenko@...ux.intel.com,
        herbert@...dor.apana.org.au, bhe@...hat.com, xemul@...allels.com,
        joro@...tes.org, x86@...nel.org, mingo@...hat.com,
        msalter@...hat.com, ross.zwisler@...ux.intel.com,
        dyoung@...hat.com, thomas.lendacky@....com, jroedel@...e.de,
        keescook@...omium.org, toshi.kani@....com,
        mathieu.desnoyers@...icios.com, devel@...uxdriverproject.org,
        tglx@...utronix.de, mchehab@...nel.org, iamjoonsoo.kim@....com,
        labbott@...oraproject.org, tony.luck@...el.com,
        alexandre.bounine@....com, kuleshovmail@...il.com,
        linux-kernel@...r.kernel.org, mcgrof@...nel.org,
        linux-crypto@...r.kernel.org, pbonzini@...hat.com,
        akpm@...ux-foundation.org, davem@...emloft.net
Subject: Re: [RFC PATCH v1 01/28] kvm: svm: Add support for additional SVM
 NPF error codes

On Mon, Aug 22, 2016 at 07:23:44PM -0400, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@....com>
> 
> AMD hardware adds two additional bits to aid in nested page fault handling.
> 
> Bit 32 - NPF occurred while translating the guest's final physical address
> Bit 33 - NPF occurred while translating the guest page tables
> 
> The guest page tables fault indicator can be used as an aid for nested
> virtualization. Using V0 for the host, V1 for the first level guest and
> V2 for the second level guest, when both V1 and V2 are using nested paging
> there are currently a number of unnecessary instruction emulations. When
> V2 is launched shadow paging is used in V1 for the nested tables of V2. As
> a result, KVM marks these pages as RO in the host nested page tables. When
> V2 exits and we resume V1, these pages are still marked RO.
> 
> Every nested walk for a guest page table is treated as a user-level write
> access and this causes a lot of NPFs because the V1 page tables are marked
> RO in the V0 nested tables. While executing V1, when these NPFs occur KVM
> sees a write to a read-only page, emulates the V1 instruction and unprotects
> the page (marking it RW). This patch looks for cases where we get a NPF due
> to a guest page table walk where the page was marked RO. It immediately
> unprotects the page and resumes the guest, leading to far fewer instruction
> emulations when nested virtualization is used.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/include/asm/kvm_host.h |   11 ++++++++++-
>  arch/x86/kvm/mmu.c              |   20 ++++++++++++++++++--
>  arch/x86/kvm/svm.c              |    2 +-
>  3 files changed, 29 insertions(+), 4 deletions(-)

FWIW: Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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