linux-kernel - Re: [PATCH] x86: Drop RDPID macro to discourage use of RDPID in the kernel

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Message-ID: <20201028085405.GW2628@hirez.programming.kicks-ass.net>
Date:   Wed, 28 Oct 2020 09:54:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Drop RDPID macro to discourage use of RDPID in the
 kernel

On Tue, Oct 27, 2020 at 02:45:32PM -0700, Sean Christopherson wrote:
> Drop the GAS-compatible RDPID macro as RDPID shouldn't be used in the
> kernel, excepting the vDSO, which handcodes RDPID in inline asm.  RDPID
> is unsafe in the kernel because KVM loads guest's TSC_AUX on VM-entry
> and may not restore the host's value until the CPU returns to userspace.
> 
> See commit 6a3ea3e68b8a ("x86/entry/64: Do not use RDPID in paranoid
> entry to accomodate KVM") for details.

Well, remove it because there is no (current) user, OK. But if there's a
compelling kernel side use for it, KVM can go suck rocks.