Re: [PATCH v3 0/5] Qemu SEV-ES guest support

[Dr. David Alan Gilbert]

* Tom Lendacky (thomas.lendacky@amd.com) wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> This patch series provides support for launching an SEV-ES guest.
> 
> Secure Encrypted Virtualization - Encrypted State (SEV-ES) expands on the
> SEV support to protect the guest register state from the hypervisor. See
> "AMD64 Architecture Programmer's Manual Volume 2: System Programming",
> section "15.35 Encrypted State (SEV-ES)" [1].
> 
> In order to allow a hypervisor to perform functions on behalf of a guest,
> there is architectural support for notifying a guest's operating system
> when certain types of VMEXITs are about to occur. This allows the guest to
> selectively share information with the hypervisor to satisfy the requested
> function. The notification is performed using a new exception, the VMM
> Communication exception (#VC). The information is shared through the
> Guest-Hypervisor Communication Block (GHCB) using the VMGEXIT instruction.
> The GHCB format and the protocol for using it is documented in "SEV-ES
> Guest-Hypervisor Communication Block Standardization" [2].
> 
> The main areas of the Qemu code that are updated to support SEV-ES are
> around the SEV guest launch process and AP booting in order to support
> booting multiple vCPUs.
> 
> There are no new command line switches required. Instead, the desire for
> SEV-ES is presented using the SEV policy object. Bit 2 of the SEV policy
> object indicates that SEV-ES is required.
> 
> The SEV launch process is updated in two ways. The first is that a the
> KVM_SEV_ES_INIT ioctl is used to initialize the guest instead of the
> standard KVM_SEV_INIT ioctl. The second is that before the SEV launch
> measurement is calculated, the LAUNCH_UPDATE_VMSA SEV API is invoked for
> each vCPU that Qemu has created. Once the LAUNCH_UPDATE_VMSA API has been
> invoked, no direct changes to the guest register state can be made.
> 
> AP booting poses some interesting challenges. The INIT-SIPI-SIPI sequence
> is typically used to boot the APs. However, the hypervisor is not allowed
> to update the guest registers. For the APs, the reset vector must be known
> in advance. An OVMF method to provide a known reset vector address exists
> by providing an SEV information block, identified by UUID, near the end of
> the firmware [3]. OVMF will program the jump to the actual reset vector in
> this area of memory. Since the memory location is known in advance, an AP
> can be created with the known reset vector address as its starting CS:IP.
> The GHCB document [2] talks about how SMP booting under SEV-ES is
> performed. SEV-ES also requires the use of the in-kernel irqchip support
> in order to minimize the changes required to Qemu to support AP booting.

Some random thoughts:
  a) Is there something that explicitly disallows SMM?
  b) I think all the interfaces you're using are already defined in
Linux header files - even if the code to implement them isn't actually
upstream in the kernel yet (the launch_update in particular) - we
normally wait for the kernel interface to be accepted before taking the
QEMU patches, but if the constants are in the headers already I'm not
sure what the rule is.
  c) What happens if QEMU reads the register values from the state if
the guest is paused - does it just see junk?  I'm just wondering if you
need to add checks in places it might try to.

Dave

> [1] https://www.amd.com/system/files/TechDocs/24593.pdf
> [2] https://developer.amd.com/wp-content/resources/56421.pdf
> [3] 30937f2f98c4 ("OvmfPkg: Use the SEV-ES work area for the SEV-ES AP reset 
> vector")
>     
> https://github.com/tianocore/edk2/commit/30937f2f98c42496f2f143fe8374ae7f7e684847
> 
> ---
> 
> These patches are based on commit:
> d0ed6a69d3 ("Update version for v5.1.0 release")
> 
> (I tried basing on the latest Qemu commit, but I was having build issues
> that level)
> 
> A version of the tree can be found at:
> https://github.com/AMDESE/qemu/tree/sev-es-v11
> 
> Changes since v2:
> - Add in-kernel irqchip requirement for SEV-ES guests
> 
> Changes since v1:
> - Fixed checkpatch.pl errors/warnings
> 
> Tom Lendacky (5):
>   sev/i386: Add initial support for SEV-ES
>   sev/i386: Require in-kernel irqchip support for SEV-ES guests
>   sev/i386: Allow AP booting under SEV-ES
>   sev/i386: Don't allow a system reset under an SEV-ES guest
>   sev/i386: Enable an SEV-ES guest based on SEV policy
> 
>  accel/kvm/kvm-all.c       |  73 ++++++++++++++++++++++++++
>  accel/stubs/kvm-stub.c    |   5 ++
>  hw/i386/pc_sysfw.c        |  10 +++-
>  include/sysemu/cpus.h     |   2 +
>  include/sysemu/hw_accel.h |   5 ++
>  include/sysemu/kvm.h      |  18 +++++++
>  include/sysemu/sev.h      |   3 ++
>  softmmu/cpus.c            |   5 ++
>  softmmu/vl.c              |   5 +-
>  target/i386/cpu.c         |   1 +
>  target/i386/kvm.c         |   2 +
>  target/i386/sev-stub.c    |   5 ++
>  target/i386/sev.c         | 105 +++++++++++++++++++++++++++++++++++++-
>  target/i386/sev_i386.h    |   1 +
>  14 files changed, 236 insertions(+), 4 deletions(-)
> 
> -- 
> 2.28.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK