linux-kernel - Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

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Message-ID: <20200623121237.GC14101@suse.de>
Date:   Tue, 23 Jun 2020 14:12:37 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <JGross@...e.com>,
        Jiri Slaby <jslaby@...e.cz>, Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)

On Tue, Jun 23, 2020 at 01:50:14PM +0200, Peter Zijlstra wrote:
> If SNP is the sole reason #VC needs to be IST, then I'd strongly urge
> you to only make it IST if/when you try and make SNP happen, not before.

It is not the only reason, when ES guests gain debug register support
then #VC also needs to be IST, because #DB can be promoted into #VC
then, and as #DB is IST for a reason, #VC needs to be too.

Besides that, I am not a fan of delegating problems I already see coming
to future-Joerg and future-Peter, but if at all possible deal with them
now and be safe later.

Regards,

	Joerg