linux-kernel - RE: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

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Date:   Wed, 29 Sep 2021 02:15:08 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Yu, Fenghua" <fenghua.yu@...el.com>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

>> 	if (!(xsave->header.xfeatures & fmask)) {
>> 		xsave->header.xfeatures |= fmask;	//<<<<<
>> 		xsaves(xsave, fmask);
>> 	}
>
> I'm not sure why the FPU state is initialized here.
>
> For updating the PASID state, it's unnecessary to init the PASID state.
>
> Maybe it is necessary in other cases?

Dave had suggested initializing feature state when it is unknown (could
be garbage).  This is my attempt to follow that guidance. I'm not confident
that my tests for "is the state in registers, in memory, or is garbage"
really capture all the cases.

-Tony