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

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Date:   Wed, 29 Sep 2021 01:56:23 +0000
From:   "Yu, Fenghua" <fenghua.yu@...el.com>
To:     "Luck, Tony" <tony.luck@...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

Hi, Tony,

> 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;
> 	}
> 
> 	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?

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

Setting the xfeatures bit plus updating the PASID state is enough
to restore the PASID state to the IA32_PASID MSR.

> 	return addr;
> }
> 
> void finish_update_one_xsave_feature(struct task_struct *tsk) {
> 	set_ti_thread_flag(task_thread_info(tsk), TIF_NEED_FPU_LOAD);
> 	if (tsk == current)				//<<<<<
> 		__cpu_invalidate_fpregs_state();	//<<<<<
> 	fpregs_unlock();
> }

Thanks.

-Fenghua