Re: [PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active

From: Borislav Petkov <bp@alien8.de>
To: Joerg Roedel <joro@8bytes.org>
Cc: x86@kernel.org, Joerg Roedel <jroedel@suse.de>,
	hpa@zytor.com, Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Juergen Gross <jgross@suse.com>,
	Kees Cook <keescook@chromium.org>,
	David Rientjes <rientjes@google.com>,
	Cfir Cohen <cfir@google.com>, Erdem Aktas <erdemaktas@google.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active
Date: Fri, 11 Jun 2021 16:05:15 +0200	[thread overview]
Message-ID: <YMNtmz6W1apXL5q+@zn.tnic> (raw)
In-Reply-To: <20210610091141.30322-3-joro@8bytes.org>

On Thu, Jun 10, 2021 at 11:11:37AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> The #VC handler only cares about IRQs being disabled while the GHCB is
> active, as it must not be interrupted by something which could cause
> another #VC while it holds the GHCB (NMI is the exception for which the
> backup GHCB is there).
> 
> Make sure nothing interrupts the code path while the GHCB is active by
> disabling IRQs in sev_es_get_ghcb() and restoring the previous irq state
> in sev_es_put_ghcb().

Why this unnecessarily complicated passing of flags back and forth?

Why not simply "sandwich" them:

	local_irq_save()
	sev_es_get_ghcb()

	...blablabla

	sev_es_put_ghcb()
	local_irq_restore();

in every call site?

What's the difference in passing *flags in and have the
get_ghcb/put_ghcb save/restore flags instead of the callers?

> -static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
> +static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state,
> +						    unsigned long *flags)
>  {
>  	struct sev_es_runtime_data *data;
>  	struct ghcb *ghcb;
>  
> +	/*
> +	 * Nothing shall interrupt this code path while holding the per-cpu
> +	 * GHCB. The backup GHCB is only for NMIs interrupting this path.

Hmm, so why aren't you accessing/setting data->ghcb_active and
data->backup_ghcb_active safely using cmpxchg() if this path can be
interrupted by an NMI?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette