Re: [PATCH v13 00/12] Add AMD SEV guest live migration support

From: Steve Rutherford <srutherford@google.com>
To: Ashish Kalra <Ashish.Kalra@amd.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@suse.de>,
	Tom Lendacky <thomas.lendacky@amd.com>, X86 ML <x86@kernel.org>,
	KVM list <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Venu Busireddy <venu.busireddy@oracle.com>,
	Brijesh Singh <brijesh.singh@amd.com>
Subject: Re: [PATCH v13 00/12] Add AMD SEV guest live migration support
Date: Fri, 16 Apr 2021 14:43:48 -0700	[thread overview]
Message-ID: <CABayD+dGWWha8opC7rFgNYs=bgWbohE+ngTRfKjw12fXrT+Q+g@mail.gmail.com> (raw)
In-Reply-To: <cover.1618498113.git.ashish.kalra@amd.com>

On Thu, Apr 15, 2021 at 8:52 AM Ashish Kalra <Ashish.Kalra@amd.com> wrote:
>
> From: Ashish Kalra <ashish.kalra@amd.com>
>
> The series add support for AMD SEV guest live migration commands. To protect the
> confidentiality of an SEV protected guest memory while in transit we need to
> use the SEV commands defined in SEV API spec [1].
>
> SEV guest VMs have the concept of private and shared memory. Private memory
> is encrypted with the guest-specific key, while shared memory may be encrypted
> with hypervisor key. The commands provided by the SEV FW are meant to be used
> for the private memory only. The patch series introduces a new hypercall.
> The guest OS can use this hypercall to notify the page encryption status.
> If the page is encrypted with guest specific-key then we use SEV command during
> the migration. If page is not encrypted then fallback to default.
>
> The patch uses the KVM_EXIT_HYPERCALL exitcode and hypercall to
> userspace exit functionality as a common interface from the guest back to the
> VMM and passing on the guest shared/unencrypted page information to the
> userspace VMM/Qemu. Qemu can consult this information during migration to know
> whether the page is encrypted.
>
> This section descibes how the SEV live migration feature is negotiated
> between the host and guest, the host indicates this feature support via
> KVM_FEATURE_CPUID. The guest firmware (OVMF) detects this feature and
> sets a UEFI enviroment variable indicating OVMF support for live
> migration, the guest kernel also detects the host support for this
> feature via cpuid and in case of an EFI boot verifies if OVMF also
> supports this feature by getting the UEFI enviroment variable and if it
> set then enables live migration feature on host by writing to a custom
> MSR, if not booted under EFI, then it simply enables the feature by
> again writing to the custom MSR. The MSR is also handled by the
> userspace VMM/Qemu.
>
> A branch containing these patches is available here:
> https://github.com/AMDESE/linux/tree/sev-migration-v13
>
> [1] https://developer.amd.com/wp-content/resources/55766.PDF
>
> Changes since v12:
> - Reset page encryption status during early boot instead of just
>   before the kexec to avoid SMP races during kvm_pv_guest_cpu_reboot().

Does this series need to disable the MSR during kvm_pv_guest_cpu_reboot()?

I _think_ going into blackout during the window after restart, but
before the MSR is explicitly reenabled, would cause corruption. The
historical shared pages could be re-allocated as non-shared pages
during restart.

Steve