Messages in this thread | | | Date | Mon, 29 Jun 2015 12:48:19 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [all better] Re: regression: massive trouble with fpu rework |
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On 06/29/2015 02:35 AM, Ingo Molnar wrote: > > Indeed, I bet that makes a difference! > > I wish that 'unmasking' logic came with more comments: > > - Why do BIOSen ever mask CPUIDs? >
To work around bugs in legacy operating systems.
> - Why do we unmask the masking?
Because we don't have those specific bugs.
> - Why doesn't the kernel keep on working just fine even if certain CPUID aspects > are turned off?
Because it exercises code paths that are otherwise impossible, for example, it exposes the XSAVE capability without exposing the XSAVE information in higher CPUID leaves.
The other option would be to have a list of CPU features that should be turned off whenever the CPUID leaf maximum is too low, but it gives a better user experience to just override the BIOS capping and then we have fewer code paths in the kernel to worry about.
-hpa
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