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

On Fri, Aug 06, 2021 at 05:07:45PM +0200, Jiri Denemark wrote: > 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. Would that be reflected by the answer to KVM_GET_SUPPORTED_CPUID which is the intersection of physical CPUID and what KVM is actally willing to enable ? That ioctl would be cheap too.