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

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Message-ID: <308a72e4-6aa9-0c84-21e6-ee613eea35a8@kernel.org>
Date:   Wed, 29 Sep 2021 09:58:22 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Luck, Tony" <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     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 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:

typedef fpu__pasid_type u32;

static inline void fpu__set_pasid_in_register(const u32 *value)
{
	wrmsr(...);
}

#define DEFINE_FPU_HELPER(name) \
static inline void fpu__set_##name(const fpu__##name##_type *val) \
{ \
	fpregs_lock(); \
	if (should write in memory) { \
		->xfeatures |= XFEATURE_##name; \
		ptr = get_xsave_addr(...); \
		memcpy(ptr, val, sizeof(*val)); \
		__fpu_invalidate_fpregs_state(...); \
	} else { \
		fpu__set_##name##_in_register(val); \
	} \
}