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

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Message-Id: <e396e8d4-9221-4426-bd38-41ccac040e5f@www.fastmail.com>
Date:   Wed, 29 Sep 2021 10:48:24 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Tony Luck" <tony.luck@...el.com>
Cc:     "Dave Hansen" <dave.hansen@...el.com>,
        "Fenghua Yu" <fenghua.yu@...el.com>,
        "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>,
        "Dave Jiang" <dave.jiang@...el.com>,
        "Jacob Jun Pan" <jacob.jun.pan@...el.com>,
        "Raj Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        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



On Wed, Sep 29, 2021, at 10:07 AM, Luck, Tony wrote:
> On Wed, Sep 29, 2021 at 09:58:22AM -0700, Andy Lutomirski wrote:
>> On 9/28/21 16:10, Luck, Tony wrote:
>> > Moving beyond pseudo-code and into compiles-but-probably-broken-code.
>> > 
>> > 
>> > The intent of the functions below is that Fenghua should be able to
>> > do:
>> > 
>> > void fpu__pasid_write(u32 pasid)
>> > {
>> > 	u64 msr_val = pasid | MSR_IA32_PASID_VALID;
>> > 	struct ia32_pasid_state *addr;
>> > 
>> > 	addr = begin_update_one_xsave_feature(current, XFEATURE_PASID, true);
>> > 	addr->pasid = msr_val;
>> > 	finish_update_one_xsave_feature(current);
>> > }
>> > 
>> 
>> This gets gnarly because we would presumably like to optimize the case where
>> we can do the update directly in registers.  I wonder if we can do it with a
>> bit of macro magic in a somewhat generic way:
>
> Can we defere the optimizations until there is a use case that
> cares? The existing use case (fixing up the #GP fault by setting
> the PASID MSR) isn't performance critical.
>
> Let's just have something that is clear (or as clear as any xsave
> code can be) and correct.
>
> 

The goal would be to use the same code for CET and PKRU, I think.