Re: [PATCH] docs: Add measurement calculation details to amd-memory-encryption.txt

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jan 10, 2022 at 01:17:02PM +0200, Dov Murik wrote:
> 
> 
> On 07/01/2022 22:18, Daniel P. Berrangé wrote:
> > On Thu, Dec 16, 2021 at 11:41:27PM +0200, Dov Murik wrote:
> >>
> >>
> >> On 16/12/2021 18:09, Daniel P. Berrangé wrote:
> >>> On Thu, Dec 16, 2021 at 12:38:34PM +0200, Dov Murik wrote:
> >>>>
> >>>>
> >>>> On 14/12/2021 20:39, Daniel P. Berrangé wrote:
> >>>>> Is there any practical guidance we can give apps on the way the VMSAs
> >>>>> can be expected to be initialized ? eg can they assume essentially
> >>>>> all fields in vmcb_save_area are 0 initialized except for certain
> >>>>> ones ? Is initialization likely to vary at all across KVM or EDK2
> >>>>> vesions or something ?
> >>>>
> >>>> From my own experience, the VMSA of vcpu0 doesn't change; it is basically what QEMU
> >>>> sets up in x86_cpu_reset() (which is mostly zeros but not all).  I don't know if it
> >>>> may change in newer QEMU (machine types?) or kvm.  As for vcpu1+, in SEV-ES the
> >>>> CS:EIP for the APs is taken from a GUIDed table at the end of the OVMF image, and has
> >>>> actually changed a few months ago when the memory layout changed to support both TDX
> >>>> and SEV.
> >>>
> >>> That is an unplesantly large number of moving parts that could
> >>> potentially impact the expected state :-(  I think we need to
> >>> be careful to avoid gratuitous changes, to avoid creating a
> >>> combinatorial expansion in the number of possibly valid VMSA
> >>> blocks.
> >>>
> >>> It makes me wonder if we need to think about defining some
> >>> standard approach for distro vendors (and/or cloud vendors)
> >>> to publish the expected contents for various combinations
> >>> of their software pieces.
> >>>
> >>>>
> >>>>
> >>>> Here are the VMSAs for my 2-vcpu SEV-ES VM:
> >>>>
> >>>>
> >>>> $ hd vmsa/vmsa_cpu0.bin
> >>>
> >>> ...snipp...
> >>>
> >>> was there a nice approach / tool you used to capture
> >>> this initial state ?
> >>>
> >>
> >> I wouldn't qualify this as nice: I ended up modifying my
> >> host kernel's kvm (see patch below).  Later I wrote a
> >> script to parse that hex dump from the kernel log into
> >> proper 4096-byte binary VMSA files.
> >>
> >>
> >>
> >> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> >> index 7fbce342eec4..4e45fe37b93d 100644
> >> --- a/arch/x86/kvm/svm/sev.c
> >> +++ b/arch/x86/kvm/svm/sev.c
> >> @@ -624,6 +624,12 @@ static int sev_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
> >>                  */
> >>                 clflush_cache_range(svm->vmsa, PAGE_SIZE);
> >>
> >> +                /* dubek */
> >> +                pr_info("DEBUG_VMSA - cpu %d START ---------------\n", i);
> >> +                print_hex_dump(KERN_INFO, "DEBUG_VMSA", DUMP_PREFIX_OFFSET, 16, 1, svm->vmsa, PAGE_SIZE, true);
> >> +                pr_info("DEBUG_VMSA - cpu %d END ---------------\n", i);
> >> +                /* ----- */
> >> +
> >>                 vmsa.handle = sev->handle;
> >>                 vmsa.address = __sme_pa(svm->vmsa);
> >>                 vmsa.len = PAGE_SIZE;
> > 
> > FWIW, I made a 1% less hacky solution by writing a systemtap
> > script. It will require changing to set the line number for
> > every single kernel version, but at least it doesn't require
> > building a custom kernel
> 
> Thanks, we'll check it out.  It does require a kernel compiled with
> debug info (I assume) to be able to hook the exact line number.

On RHEL / Fedora, you should merely need to install the corresponding
-debuginfo RPM to match your running kernel.

Regards,
Daniel
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