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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@amd.com> Reviewed-by: Borislav Petkov <bp@suse.de> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com> |
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