Re: [PATCH for-9.1 v1 0/3] Add SEV/SEV-ES machine compat options for KVM_SEV_INIT2

[Paolo Bonzini <pbonzini@redhat.com>]

On Wed, Apr 10, 2024 at 1:08 AM Michael Roth <michael.roth@amd.com> wrote:
>
> These patches are also available at:
>
>   https://github.com/amdese/qemu/commits/sev-init-legacy-v1
>
> and are based on top Paolo's qemu-coco-queue branch containing the
> following patches:

A more complete version of patch 2 was already on the list, so I
queued 1 and 3 to qemu-coco-queue.

Thanks!

Paolo

>
>   [PATCH for-9.1 00/26] x86, kvm: common confidential computing subset
>   https://lore.kernel.org/all/20240322181116.1228416-1-pbonzini@redhat.com/T/
>
> Overview
> --------
>
> With the following patches applied from qemu-coco-queue:
>
>   https://lore.kernel.org/all/20240319140000.1014247-1-pbonzini@redhat.com/
>
> QEMU version 9.1+ will begin automatically making use of the new
> KVM_SEV_INIT2 API for initializing SEV and SEV-ES (and eventually, SEV-SNP)
> guests verses the older KVM_SEV_INIT/KVM_SEV_ES_INIT interfaces.
>
> However, the older interfaces would silently avoid sync'ing FPU/XSAVE state
> set by QEMU to each vCPU's VMSA prior to encryption. With KVM_SEV_INIT2,
> this state will now be synced into the VMSA, resulting in measurements
> changes and, theoretically, behaviorial changes, though the latter are
> unlikely to be seen in practice. The specific VMSA changes are documented
> in the section below for reference.
>
> This series implements machine compatibility options for SEV/SEV-ES so that
> only VMs created with QEMU 9.1+ will make use of KVM_SEV_INIT2 so that VMSA
> differences can be accounted for beforehand, and older machine types will
> continue using the older interfaces to avoid unexpected measurement
> changes.
>
> Specific VMSA changes
> ---------------------
>
> With KVM_SEV_INIT2, rather than 0, QEMU/KVM will instead begin setting the
> following fields in the VMSA before measurement/encryption:
>
>   VMSA byte offset [1032:1033] = 80 1f (MXCSR, Multimedia Control Status
>                                         Register)
>   VMSA byte offset [1040:1041] = 7f 03 (FCW, FPU/x86 Control Word)
>
> Setting FCW (FPU/x86 Control Word) to 0x37f is consistent with 11.5.7 of
> APM Volume 2. MXCSR reset state is not defined for XSAVE, but QEMU's 0x1f80
> value is consistent with machine reset state documented in APM Volume 2
> 4.2.2. As such, it is reasonable to begin including these in the VMSA
> measurement calculations.
>
> NOTE: section 11.5.7 also documents that FTW should be all 1's, whereas
>       QEMU currently sets all zeroes. Should that be changed as part of
>       this, or are there other reasons for setting 0?
>
> Thanks,
>
> Mike
>
> ----------------------------------------------------------------
> Michael Roth (3):
>       i386/sev: Add 'legacy-vm-type' parameter for SEV guest objects
>       hw/i386: Add 9.1 machine types for i440fx/q35
>       hw/i386/sev: Use legacy SEV VM types for older machine types
>
>  hw/i386/pc.c         |  5 +++++
>  hw/i386/pc_piix.c    | 13 ++++++++++++-
>  hw/i386/pc_q35.c     | 12 +++++++++++-
>  include/hw/i386/pc.h |  3 +++
>  qapi/qom.json        | 11 ++++++++++-
>  target/i386/sev.c    | 19 ++++++++++++++++++-
>  6 files changed, 59 insertions(+), 4 deletions(-)
>
>
>