Messages in this thread | | | From | Andrew Cooper <> | Subject | Re: [PATCH] x86: also disable FSRM if ERMS is disabled | Date | Tue, 11 Oct 2022 22:59:11 +0000 |
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On 11/10/2022 23:19, Luck, Tony wrote: >> That won't help because userspace will still use them since the CPUID >> flags remain set. > Even if writing the MSR did clear the CPUID bits, it wouldn't help with applications > that started before the kernel cleared the bits (assuming this was some run-time > update patch). > > Worst case scenario is that the applications don't pick the most efficient memcpy().
As hint-only bits, the kernel is free to do whatever it wants behind the scenes. The only case I'm aware of which dynamically clears fast strings is the #MC ECC poison overread erratum.
If you want to truly hide the CPUID bits, then you can use CPUID Faulting (IvyBridge and later, available to userspace), or for a few generations prior to that, the CPUID mask MSRs, but the settings do want to be uniform from boot.
~Andrew
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