Re: [PATCH] x86/sev: Make SEV_STATUS available via SYSFS

Kirill A. Shutemov
2025-03-06T08:01:22Z

On Wed, Mar 05, 2025 at 08:40:29AM -0800, Dave Hansen wrote:
> On 3/5/25 07:37, Borislav Petkov wrote:
> >> So this question boils down to whether the parsing of the bits happens
> >> in kernel- or user-space. Actually there is already parsing in
> >> kernel-space to print the status bits into the kernel log:
> >>
> >> SEV: Status: SEV SEV-ES SEV-SNP
> >>
> >> ... which is great for a quick glance without needing any tools. The
> >> user-space tools which already exist have their own parsing of the bits
> >> and for them it is much easier to consume the raw value of the
> >> SEV_STATUS MSR. See my changes to snpguest:
> >>
> >> https://github.com/virtee/snpguest/pull/88/files
> > Well, I guess we can do both:
> >
> > cat /sys/...
> >
> > SEV_STATUS(0xdeadbeef): SEV SEV-ES SEV-SNP
> >
> > So that people don't have to pick apart the MSR hex value.
> >
> >> Btw, what is the equivalent on the Intel TDX side for these feature
> >> bits?
> > There is none, AFAICT. That's why the whole discussion.
>
> TDX's history isn't as exciting as SEV.
>
> TDX guests have CPUID to tell them that they're running that way.
>
> TDX hosts are much more arcane. You can't _actually_ know that it's a
> TDX host until you actually start making successful SEAMCALLs and the
> TDX module answers them. But we fudge it by just looking at
> MSR_IA32_MKTME_KEYID_PARTITIONING at boot and assuming that anything
> with that MSR will be able to be a TDX host.
>
> We've just got X86_FEATUREs for hosts and guests:
>
> #define X86_FEATURE_TDX_HOST_PLATFORM ( 7*32+ 7)
> #define X86_FEATURE_TDX_GUEST ( 8*32+22)
>
> and that's it.
>
> Folks certainly _want_ something in sysfs to dump the TDX module version
> and so forth, but we've resisted the urge so far.

Alexey looking into exposing TDX module version in sysfs for both guest
and host.

I think it would be useful for guest to make attributes and TD_CTLS
available via sysfs. So far, we only dump them in dmesg on boot (see
564ea84c8c14).

--
Kiryl Shutsemau / Kirill A. Shutemov