linux-kernel - Re: [RFC Part2 PATCH v3 26/26] KVM: X86: Restart the guest when insn

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Date:   Thu, 14 Sep 2017 17:40:32 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        \"Radim Krčmář\" <rkrcmar@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC Part2 PATCH v3 26/26] KVM: X86: Restart the guest when
 insn_len is zero and SEV is enabled

On Mon, Jul 24, 2017 at 03:03:03PM -0500, Brijesh Singh wrote:
> On AMD platform, under certain conditions insn_len may be zero on #NPF.

"On AMD platforms... "

> This can happen if guest gets a page-fault on data access, but HW table

		" ... if a guest...  .. but the HW table walker ..."

> walker is not able to read the instruction page (e.g instuction 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.

Same question as in the previous patch: should we make this a proper
interface?

> I have encountered this issue when running kernbench inside the guest.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/kvm/mmu.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index ccb70b8..be41ad0 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4850,6 +4850,23 @@ 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 platform, under certain conditions insn_len may be zero on #NPF.

Same typos as above.

> +	 * This can happen if guest gets a page-fault on data access, but HW table
> +	 * walker is not able to read the instruction page (e.g instuction page
> +	 * is not present).
> +	 *
> +	 * Typically, when insn_len is zero, x86_emulate_instruction() walks the
> +	 * guest page table and fetches the instruction bytes. When SEV is active,
> +	 * the guest memory is encrypted with guest key hence we will not able to
> +	 * fetch the instruction bytes. In those cases we simply restart the guest.
> +	 */
> +	if (unlikely(!insn_len)) {
> +		if (kvm_x86_ops->memory_encryption_enabled &&
> +			kvm_x86_ops->memory_encryption_enabled(vcpu))

Align vertically.


-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--