[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen
On Wed, Jul 06, 2022 at 08:17:41AM +0200, Jan Beulich wrote: > Sure, but that alone won't help. Well, the MTRR code looks at X86_FEATURE_MTRR. If Xen doesn't expose the MTRRs, then that bit should be clear in the CPUID the guest sees. So in that case, you could test X86_FEATURE_XENPV at the end of mtrr_bp_init() and not disable PAT if running as a PV guest. Would that work? > There's a beneficial side effect of running through pat_disable(): > That way pat_init() will bail right away. Without that I'd need to > further special case things there (as under Xen/PV PAT must not be > written, only read) We have wrmsr_safe for that. > Any decent hypervisor will allow overriding CPUID, so in principle > I'd expect any to permit disabling MTRR to leave a guest to use > the (more modern and less cumbersome) PAT alone. So I'm being told that it would be generally beneficial for all kinds of virtualization solutions to be able to support PAT only, without MTRRs so it would be interesting to see how ugly it would become to decouple PAT from MTRRs in Linux... Thx. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette
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