linux-kernel - Re: [Xen-devel] [PATCH] x86/amd: fix crash as Xen Dom0 on AMD Trinity systems

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Date:	Wed, 30 May 2012 18:33:34 -0400
From:	Konrad Rzeszutek Wilk <konrad@...nok.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jacob Shin <jacob.shin@....com>,
	Andre Przywara <andre.przywara@....com>, jeremy@...p.org,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	Jan Beulich <JBeulich@...e.com>, mingo@...e.hu,
	tglx@...utronix.de
Subject: Re: [Xen-devel] [PATCH] x86/amd: fix crash as Xen Dom0 on AMD Trinity systems

> > Now, someone probably needs to paravirt the *safe_regs variants in case
> > something else decides to use them. I don't know what to do here, do I
> > want more paravirt code in there? No. I guess if this is done carefully
> > and cleanly, then it should be ok but it can't be done like that - it
> > needs to adhere to the current pv_cpu_ops thing which is already there.

Using the native variant seems the right thing to do.
> > 
> 
> I thought I was being told that Xen would trap and emulate the
> rdmsr/wrmsr instructions.  I guess they don't want to do that for the

It does.
> handful of performance-sensitive MSRs there are, but those don't use the
> *_regs variants.

The underlaying issue (as I understand) was that .rdmsr_regs
(and the corresponding write) was NULL and that caused the crash.
This tiny patch should do it:

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 75f33b2..e74df95 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1116,7 +1116,10 @@ static const struct pv_cpu_ops xen_cpu_ops __initconst = {
 	.wbinvd = native_wbinvd,
 
 	.read_msr = native_read_msr_safe,
+	.rdmsr_regs = native_rdmsr_safe_regs,
 	.write_msr = xen_write_msr_safe,
+	.wrmsr_regs = native_wrmsr_safe_regs,
+
 	.read_tsc = native_read_tsc,
 	.read_pmc = native_read_pmc,
 
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