[libvirt PATCH 0/3] Invalidate the cpu flags cache on changes of kernel command line

Jiri Denemark jdenemar at redhat.com
Fri Aug 6 15:07:45 UTC 2021

On Thu, Aug 05, 2021 at 14:50:51 +0100, Daniel P. Berrangé wrote:
> On Thu, Aug 05, 2021 at 03:36:37PM +0200, Tim Wiederhake wrote:
> > The kernel command line can contain settings affecting the availability
> > of cpu features, eg. "tsx=on". This series adds the kernel command line
> > to the cpu flags cache and declares the cache invalid if the current
> > kernel command line differs.
> Multiple things can change the CPU features. kernel version,
> microcode version, bios settings change, kernel command line. We've
> been playing whack-a-mole in cache invalidation for ages adding ever
> more criteria for things which have side effects on CPU features
> available.
> Running the CPUID instruction is cheap. Could we directly query the
> set of host CPUID leaves we care about, and compare that, and
> potentially even get rid of some of the other checks we have ?

I guess it could help in some cases, but we wouldn't be able to drop
some of the existing checks anyway. Because the settings usually do not
result in the CPU dropping a particular bit from CPUID, the feature just
becomes unusable by reporting a failure when used. So the settings would
only be reflected in what features QEMU can enable on the host. Although
checking CPUID might be enough for TSX, checking the command line is
helpful in other cases.

I'm afraid the only 100% correct solution would be to stop caching CPU
data at all and always probe QEMU (for host CPU model expansion only and
drop all the cache if the set of features changes), but this might be
quite expensive. On the other hand, we would need to do it only for KVM,
which means a single (or two for 32b vs 64b) on a host..


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