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

[Borislav Petkov]

On Mon, Aug 22, 2016 at 07:23:44PM -0400, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> 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@xxxxxxx>
> ---
>  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@xxxxxxx>

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