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

Ashish Kalra
2021-04-19T14:41:07Z

On Fri, Apr 16, 2021 at 02:43:48PM -0700, Steve Rutherford wrote:
> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FAMDESE%2Flinux%2Ftree%2Fsev-migration-v13&amp;data=04%7C01%7CAshish.Kalra%40amd.com%7C7bee6d5c907b46d0998508d90120ce2d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637542063133830260%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=FkKrciL41GDNyNDqrPMVblRa%2FaReogW4OzhbYaSYs04%3D&amp;reserved=0
> >
> > [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdeveloper.amd.com%2Fwp-content%2Fresources%2F55766.PDF&amp;data=04%7C01%7CAshish.Kalra%40amd.com%7C7bee6d5c907b46d0998508d90120ce2d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637542063133830260%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=%2FLFBR9ean0acMmR8WTLUHZsAynYPRAa7%2FeZEVVdpCo8%3D&amp;reserved=0
> >
> > 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()?
>

Yes, i think that will make sense, it will be similar to the first time
VM boot where the MSR will be disabled till it is enabled at early
kernel boot. I will add this to the current patch series.

Thanks,
Ashish

> 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