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

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Date:   Wed, 29 Sep 2021 10:07:54 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
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 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.

-Tony