Re: [Part2 PATCH v9 38/38] KVM: X86: Restart the guest when insn_len is zero and SEV is enabled

Jim Mattson
2019-02-01T20:21:33Z

On Mon, Dec 4, 2017 at 5:07 PM Brijesh Singh <brijesh.singh@amd.com> wrote:
>
> On AMD platforms, under certain conditions insn_len may be zero on #NPF.
> This can happen if a guest gets a page-fault on data access but the HW
> table walker is not able to read the instruction page (e.g instruction
> page is not present in memory).
>
> Typically, when insn_len is zero, x86_emulate_instruction() walks the
> guest page table and fetches the instruction bytes from guest memory.
> When SEV is enabled, the guest memory is encrypted with guest-specific
> key hence hypervisor will not able to fetch the instruction bytes.
> In those cases we simply restart the guest.
>
> I have encountered this issue when running kernbench inside the guest.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: x86@kernel.org
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/kvm/mmu.c | 10 ++++++++++
> arch/x86/kvm/svm.c | 6 ++++--
> 2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index e5e66e5c6640..d5e5dbd0e5ad 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4950,6 +4950,16 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
> if (mmio_info_in_cache(vcpu, cr2, direct))
> emulation_type = 0;
> emulate:
> + /*
> + * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
> + * This can happen if a guest gets a page-fault on data access but the HW
> + * table walker is not able to read the instruction page (e.g instruction
> + * page is not present in memory). In those cases we simply restart the
> + * guest.
> + */
> + if (unlikely(insn && !insn_len))
> + return 1;
> +

How does this work, for instance, with MMIO at CPL3 with SMAP enabled?