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

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.