linux-kernel - Re: [PATCH v3 04/10] x86/fpu/xstate: Correct guest fpstate size calculation

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Message-ID: <Z8ux4Df3o2PPxXey@intel.com>
Date: Sat, 8 Mar 2025 10:56:32 +0800
From: Chao Gao <chao.gao@...el.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: <tglx@...utronix.de>, <x86@...nel.org>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<peterz@...radead.org>, <rick.p.edgecombe@...el.com>,
	<weijiang.yang@...el.com>, <john.allen@....com>, <bp@...en8.de>
Subject: Re: [PATCH v3 04/10] x86/fpu/xstate: Correct guest fpstate size
 calculation

On Fri, Mar 07, 2025 at 09:53:40AM -0800, Dave Hansen wrote:
>On 3/7/25 08:41, Chao Gao wrote:
>> Note that this issue does not cause any functional problems because the
>> guest fpstate is allocated using vmalloc(), which aligns the size to a
>> full page, providing enough space for all existing supervisor components.
>> On Emerald Rapids CPUs, the guest fpstate after this correction is ~2880
>> bytes.
>
>How about we move up the fpstate pointers at allocation time so they
>just scrape the end of the vmalloc buffer? Basically, move the
>page-alignment padding to the beginning of the first page instead of the
>end of the last page.

That sounds like a good way to detect similar errors and might be helpful for
all other vmalloc'ed buffers.

I can try to implement this for the fpstate pointers. The patch will be put
at the end of the series or even in a separate series.