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

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Message-ID: <YVOg7zgpdQlc7Zjt@agluck-desk2.amr.corp.intel.com>
Date:   Tue, 28 Sep 2021 16:10:39 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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

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);
}

So here's the two new functions that would be added to
arch/x86/kernel/fpu/xstate.c

----

void *begin_update_one_xsave_feature(struct task_struct *tsk,
                                     enum xfeature xfeature, bool full)
{
        struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
        struct xregs_state *xinit = &init_fpstate.xsave;
        u64 fmask = 1ull << xfeature;
        void *addr;

        BUG_ON(!(xsave->header.xcomp_bv & fmask));

        fpregs_lock();

        addr = __raw_xsave_addr(xsave, xfeature);

        if (full || tsk != current) {
                memcpy(addr, __raw_xsave_addr(xinit, xfeature), xstate_sizes[xfeature]);
                goto out;
        }

	/* could optimize some cases where xsaves() isn't fastest option */
        if (!(xsave->header.xfeatures & fmask))
                xsaves(xsave, fmask);

out:
        xsave->header.xfeatures |= fmask;
        return addr;
}

void finish_update_one_xsave_feature(struct task_struct *tsk)
{
        set_ti_thread_flag(task_thread_info(tsk), TIF_NEED_FPU_LOAD);
        fpregs_unlock();
}

----

-Tony