Messages in this thread | | | Subject | Re: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES | From | Dave Hansen <> | Date | Tue, 1 Mar 2016 16:45:41 -0800 |
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On 03/01/2016 04:34 PM, Yu-cheng Yu wrote: > On Tue, Mar 01, 2016 at 03:56:12PM -0800, Dave Hansen wrote: >> On 02/29/2016 09:42 AM, Yu-cheng Yu wrote: >>> - setup_clear_cpu_cap(X86_FEATURE_XSAVES); >>> + if (!config_enabled(CONFIG_X86_64)) >>> + setup_clear_cpu_cap(X86_FEATURE_XSAVES); >>> } >> >> I think we need a much better explanation of this for posterity. Why >> are we not supporting this now, and what would someone have to do in the >> future in order to enable it? >> > If anyone is using this newer feature, then that user is most likely using > a 64-bit capable processor and a 64-bit kernel. The intention here is to > take out the complexity and any potential of error. If the user removes > the restriction and builds a private kernel, it should work but we have > not checked all possible combinations. I will put these in the comments.
A user can go download a 32-bit version of Ubuntu or Debian and install it on a 64-bit processor today. It's a very easy mistake to make when downloading the install CD.
In any case, I don't have a _problem_ with leaving i386 in the dust here. I just want us to be very explicit about what we are doing.
>>> + /* >>> + * Make it clear that XSAVES supervisor states are not yet >>> + * implemented should anyone expect it to work by changing >>> + * bits in XFEATURE_MASK_* macros and XCR0. >>> + */ >>> + WARN_ONCE((xfeatures_mask & XFEATURE_MASK_SUPERVISOR), >>> + "x86/fpu: XSAVES supervisor states are not yet implemented.\n"); >>> + >>> cr4_set_bits(X86_CR4_OSXSAVE); >>> xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask); >>> } >> >> Let's also do a: >> >> xfeatures_mask &= ~XFEATURE_MASK_SUPERVISOR; >> >> Otherwise, we have a broken system at the moment. >> > Currently, if anyone sets any supervisor state in xfeatures_mask, the > kernel prints out the warning then goes into a protection fault. > That is a very strong indication to the user. Do we want to mute it?
By "goes into a protection fault", do you mean that it doesn't boot?
I'd just rather we put the kernel in a known-safe configuration (masking supervisor state out of xfeatures_mask) rather than rely on the general protection fault continuing to be generated by whatever is generating it.
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