Re: [PATCH v7 04/26] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL_DYNAMIC xfeature set

[Yang, Weijiang]

On 11/28/2023 9:46 AM, Edgecombe, Rick P wrote:
> On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> > Define new XFEATURE_MASK_KERNEL_DYNAMIC set including the features
> > can be
> > optionally enabled by kernel components, i.e., the features are
> > required by
> > specific kernel components.
> The above is a bit tough to parse. Does any of this seem clearer?
>
> Define a new XFEATURE_MASK_KERNEL_DYNAMIC mask to specify the features
> that can be optionally enabled by kernel components. This is similar to
> XFEATURE_MASK_KERNEL_DYNAMIC in that it contains optional xfeatures
> that can allows the FPU buffer to be dynamically sized. The difference
> is that the KERNEL variant contains supervisor features and will be
> enabled by kernel components that need them, and not directly by the
> user.

Definitely the wording is much better,  I'll apply it to next version, thanks a lot!

> >   Currently it's used by KVM to configure guest
> > dedicated fpstate for calculating the xfeature and fpstate storage
> > size etc.
> >
> > The kernel dynamic xfeatures now only contain XFEATURE_CET_KERNEL,
> > which is
> > supported by host as they're enabled in xsaves/xrstors operating
> > xfeature set
> > (XCR0 | XSS), but the relevant CPU feature, i.e., supervisor shadow
> > stack, is
> > not enabled in host kernel so it can be omitted for normal fpstate by
> > default.
> >
> > Remove the kernel dynamic feature from
> > fpu_kernel_cfg.default_features so that
> > the bits in xstate_bv and xcomp_bv are cleared and xsaves/xrstors can
> > be
> > optimized by HW for normal fpstate.
> Thanks for breaking these into small patches.