linux-kernel - Re: [PATCH v3 4/7] x86/sev-es: Run #VC handler in plain IRQ state

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Date:   Tue, 8 Jun 2021 13:58:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 4/7] x86/sev-es: Run #VC handler in plain IRQ state

On Tue, Jun 08, 2021 at 11:54:36AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Use irqentry_enter() and irqentry_exit() to track the runtime state of
> the #VC handler. The reason it ran in NMI mode was solely to make sure
> nothing interrupts the handler while the GHCB is in use.
> 
> This is handled now in sev_es_get/put_ghcb() directly, so there is no
> reason the #VC handler can not run in normal IRQ mode and enjoy the
> benefits like being able to send signals.

You sure?

So #VC cannot happen with IRQs disabled?

	raw_spin_lock_irq(&my_lock);
	<#VC>
		raw_spin_lock_irqsave(&my_lock); // whoopsie

Every exception that can happen with IRQs disabled must be NMI like.

Again, what you seem to want is to split the handler in a from-user and
from-kernel way, just like we did with #DB and MCE. See how
exc_debug_user() is IRQ-like and can send signals, while
exc_debug_kernel() is NMI like and can not.