linux-kernel - RE: [PATCH v12 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions

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Date:   Tue, 19 May 2020 12:20:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sasha Levin' <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>, "luto@...nel.org" <luto@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>,
        "chang.seok.bae@...el.com" <chang.seok.bae@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH v12 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions
 in helper functions

From: Sasha Levin
> Sent: 18 May 2020 21:25
> Thank you for taking the time to review this.
> 
> On Mon, May 18, 2020 at 08:20:08PM +0200, Thomas Gleixner wrote:
> >Sasha Levin <sashal@...nel.org> writes:
> >> +unsigned long x86_gsbase_read_cpu_inactive(void)
> >> +{
> >> +	unsigned long gsbase;
> >> +
> >> +	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
> >> +		bool need_restore = false;
> >> +		unsigned long flags;
> >> +
> >> +		/*
> >> +		 * We read the inactive GS base value by swapping
> >> +		 * to make it the active one. But we cannot allow
> >> +		 * an interrupt while we switch to and from.
> >> +		 */
> >> +		if (!irqs_disabled()) {
> >> +			local_irq_save(flags);
> >> +			need_restore = true;
> >> +		}
> >> +
> >> +		native_swapgs();
> >> +		gsbase = rdgsbase();
> >> +		native_swapgs();

Does local_irq_save() even do anything useful here.
You need to actually execute CLI, not just set a
flag that indicates interrupts shouldn't happen.
(Which is what I think local_irq_save() might do.)

You also (probably) need to disable NMIs.

	David

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