Re: [PATCH v2 2/2] x86/cpuid: support LFENCE always serializing CPUID bit

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.04.2021 16:47, Roger Pau Monne wrote:
> AMD Milan (Zen3) CPUs have an LFENCE Always Serializing CPUID bit in
> leaf 80000021.eax. Previous AMD versions used to have a user settable
> bit in DE_CFG MSR to select whether LFENCE was dispatch serializing,
> which Xen always attempts to set. The forcefully always on setting is
> due to the addition of SEV-SNP so that a VMM cannot break the
> confidentiality of a guest.
> 
> In order to support this new CPUID bit move the LFENCE_DISPATCH
> synthetic CPUID bit to map the hardware bit (leaving a hole in the
> synthetic range) and either rely on the bit already being set by the
> native CPUID output, or attempt to fake it in Xen by modifying the
> DE_CFG MSR. This requires adding one more entry to the featureset to
> support leaf 80000021.eax.
> 
> The bit is exposed to guests by default if the underlying hardware
> supports leaf 80000021, as a way to signal that LFENCE is always
> serializing. Hardware that doesn't have the leaf might also get the
> bit set because Xen has performed the necessary arrangements, but
> that's not yet exposed to guests. Note that Xen doesn't allow guests
> to change the DE_CFG value, so once set by Xen LFENCE will always be
> serializing.
> 
> Note that the access to DE_CFG by guests is left as-is: reads will
> unconditionally return LFENCE_SERIALISE bit set, while writes are
> silently dropped.
> 
> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> ---
> Note this doesn't yet expose the bit on hardware that doesn't support
> leaf 80000021. It's still TBD whether we want to hardcode this support
> manually, or instead rely on a more general approach like the one
> suggested by the shrink max CPUID leaf patch from Jan.

I'd like to give Andrew a day or two more to respond there in case he
continues to see an issue, before I commit that with your R-b and this
one here. I'll assume you'll subsequently take care of that missing
piece then - if not, i.e. if e.g. I should, please let me know.

Jan