linux-kernel - Re: [PATCH v2 3/6] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <21824e7a-092e-40f8-a0f0-972c92ae3900@intel.com>
Date: Tue, 4 Mar 2025 14:37:00 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Chao Gao <chao.gao@...el.com>, tglx@...utronix.de, x86@...nel.org,
 seanjc@...gle.com, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: peterz@...radead.org, rick.p.edgecombe@...el.com, mlevitsk@...hat.com,
 weijiang.yang@...el.com, john.allen@....com
Subject: Re: [PATCH v2 3/6] x86/fpu/xstate: Introduce
 XFEATURE_MASK_KERNEL_DYNAMIC xfeature set

Subjects should ideally be written without large identifiers:

	Subject: x86/fpu/xstate: Introduce dynamic kernel features

On 11/26/24 02:17, Chao Gao wrote:
> 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.

I'm really confused why this changelog hunk is here.

Right now, all kernel XSAVE buffers have the same supervisor xfeatures.
This introduces the idea that they can have different xfeature sets.

The _purpose_ of this is to save space when only some FPUs need a given
feature. This saves space in 'struct fpu'. It probably doesn't actually
make XSAVE/XRSTOR any faster because the init optimization is very
likely to be in place for these features.

I'm not sure what point the changelog was trying to make about xstate_bv
and xcomp_bv.  xstate_bv[12] would have been 0 if the feature was in its
init state.