linux-kernel - Re: [PATCH] x86/sev: Fully map the #VC exception stacks

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Message-ID: <YVcNUzrtRqnsH6yj@hirez.programming.kicks-ass.net>
Date:   Fri, 1 Oct 2021 15:29:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Borislav Petkov <bp@...en8.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH] x86/sev: Fully map the #VC exception stacks

On Fri, Oct 01, 2021 at 03:00:38PM +0200, Joerg Roedel wrote:
> On Fri, Oct 01, 2021 at 02:58:28PM +0200, Borislav Petkov wrote:
> > On Fri, Oct 01, 2021 at 01:50:24PM +0200, Joerg Roedel wrote:
> > > Yeah, I think the right fix is to export cea_map_percpu_pages() and move
> > > the cea_map_stack() macro to a header and use it to map the VC stacks.
> > 
> > I'll do you one better: Put the #VC stack mapping into
> > percpu_setup_exception_stacks(), where it naturally belongs and where
> > the other stacks are being mapped instead of doing everything "by hand"
> > like now and exporting random helpers.
> 
> The VC stack is only allocated and mapped when SEV-ES is detected, so
> they can't always be mapped by generic code.

It's just a few pages per cpu, is that worth it? Why not have it
unconditinoally allocated in the right place when it finds the CPU is
SEV capable?