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From: Joerg Roedel <joro@8bytes.org>
To: Borislav Petkov <bp@alien8.de>
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 16:20:36 +0200	[thread overview]
Message-ID: <YMNxNEb/T3iF4TG8@8bytes.org> (raw)
In-Reply-To: <YMNtmz6W1apXL5q+@zn.tnic>

On Fri, Jun 11, 2021 at 04:05:15PM +0200, Borislav Petkov wrote:
> On Thu, Jun 10, 2021 at 11:11:37AM +0200, Joerg Roedel wrote:
> Why not simply "sandwich" them:
> 
> 	local_irq_save()
> 	sev_es_get_ghcb()
> 
> 	...blablabla
> 
> 	sev_es_put_ghcb()
> 	local_irq_restore();
> 
> in every call site?

I am not a fan of this, because its easily forgotten to add
local_irq_save()/local_irq_restore() calls around those. Yes, we can add
irqs_disabled() assertions to the functions, but we can as well just
disable/enable IRQs in them. Only the previous value of EFLAGS.IF needs
to be carried from one function to the other.

> 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?

Using cmpxchg is not necessary here. It is all per-cpu data, so local to
the current cpu. If an NMI happens anywhere in sev_es_get_ghcb() it can
still use the GHCB, because the interrupted #VC handler will not start
writing to it before sev_es_get_ghcb() returned.

Problems only come up when one path starts writing to the GHCB, but that
happens long after it is marked active.

Regards,

	Joerg

WARNING: multiple messages have this Message-ID
From: Joerg Roedel <joro@8bytes.org>
To: Borislav Petkov <bp@alien8.de>
Cc: kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	virtualization@lists.linux-foundation.org,
	Arvind Sankar <nivedita@alum.mit.edu>,
	hpa@zytor.com, Jiri Slaby <jslaby@suse.cz>,
	x86@kernel.org, David Rientjes <rientjes@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Joerg Roedel <jroedel@suse.de>, Kees Cook <keescook@chromium.org>,
	Cfir Cohen <cfir@google.com>,
	linux-coco@lists.linux.dev, Andy Lutomirski <luto@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Juergen Gross <jgross@suse.com>, Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Erdem Aktas <erdemaktas@google.com>
Subject: Re: [PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active
Date: Fri, 11 Jun 2021 16:20:36 +0200	[thread overview]
Message-ID: <YMNxNEb/T3iF4TG8@8bytes.org> (raw)
In-Reply-To: <YMNtmz6W1apXL5q+@zn.tnic>

On Fri, Jun 11, 2021 at 04:05:15PM +0200, Borislav Petkov wrote:
> On Thu, Jun 10, 2021 at 11:11:37AM +0200, Joerg Roedel wrote:
> Why not simply "sandwich" them:
> 
> 	local_irq_save()
> 	sev_es_get_ghcb()
> 
> 	...blablabla
> 
> 	sev_es_put_ghcb()
> 	local_irq_restore();
> 
> in every call site?

I am not a fan of this, because its easily forgotten to add
local_irq_save()/local_irq_restore() calls around those. Yes, we can add
irqs_disabled() assertions to the functions, but we can as well just
disable/enable IRQs in them. Only the previous value of EFLAGS.IF needs
to be carried from one function to the other.

> 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?

Using cmpxchg is not necessary here. It is all per-cpu data, so local to
the current cpu. If an NMI happens anywhere in sev_es_get_ghcb() it can
still use the GHCB, because the interrupted #VC handler will not start
writing to it before sev_es_get_ghcb() returned.

Problems only come up when one path starts writing to the GHCB, but that
happens long after it is marked active.

Regards,

	Joerg
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

  reply	other threads:[~2021-06-11 14:20 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-10  9:11 [PATCH v4 0/6] x86/sev-es: Fixes for SEV-ES Guest Support Joerg Roedel
2021-06-10  9:11 ` Joerg Roedel
2021-06-10  9:11 ` [PATCH v4 1/6] x86/sev-es: Fix error message in runtime #VC handler Joerg Roedel
2021-06-10  9:11   ` Joerg Roedel
2021-06-10  9:11 ` [PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active Joerg Roedel
2021-06-10  9:11   ` Joerg Roedel
2021-06-11 14:05   ` Borislav Petkov
2021-06-11 14:05     ` Borislav Petkov
2021-06-11 14:20     ` Joerg Roedel [this message]
2021-06-11 14:20       ` Joerg Roedel
2021-06-11 14:34       ` Borislav Petkov
2021-06-11 14:34         ` Borislav Petkov
2021-06-10  9:11 ` [PATCH v4 3/6] x86/sev-es: Split up runtime #VC handler for correct state tracking Joerg Roedel
2021-06-10  9:11   ` Joerg Roedel
2021-06-10 10:19   ` Peter Zijlstra
2021-06-10 10:19     ` Peter Zijlstra
2021-06-10 11:30     ` Joerg Roedel
2021-06-10 11:30       ` Joerg Roedel
2021-06-10  9:11 ` [PATCH v4 4/6] x86/insn-eval: Make 0 a valid RIP for insn_get_effective_ip() Joerg Roedel
2021-06-10  9:11   ` Joerg Roedel
2021-06-10  9:11 ` [PATCH v4 5/6] x86/insn: Extend error reporting from insn_fetch_from_user[_inatomic]() Joerg Roedel
2021-06-10  9:11   ` Joerg Roedel
2021-06-10  9:11 ` [PATCH v4 6/6] x86/sev-es: Propagate #GP if getting linear instruction address failed Joerg Roedel
2021-06-10  9:11   ` Joerg Roedel

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