linux-kernel - Re: [PATCH v2 3/3] KVM: VMX: enable guest access to LMCE related MSRs

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Message-ID: <20160616145546.GJ18662@thinpad.lan.raisama.net>
Date:	Thu, 16 Jun 2016 11:55:46 -0300
From:	Eduardo Habkost <ehabkost@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Haozhong Zhang <haozhong.zhang@...el.com>, kvm@...r.kernel.org,
	rkrcmar@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	Boris Petkov <bp@...e.de>, Tony Luck <tony.luck@...el.com>,
	Andi Kleen <andi.kleen@...el.com>,
	Ashok Raj <ashok.raj@...el.com>, libvir-list@...hat.com,
	Jiri Denemark <jdenemar@...hat.com>
Subject: Re: [PATCH v2 3/3] KVM: VMX: enable guest access to LMCE related MSRs

On Thu, Jun 16, 2016 at 12:04:50PM +0200, Paolo Bonzini wrote:
> On 16/06/2016 08:05, Haozhong Zhang wrote:
> > From: Ashok Raj <ashok.raj@...el.com>
> > 
> > On Intel platforms, this patch adds LMCE to KVM MCE supported
> > capabilities and handles guest access to LMCE related MSRs.
> > 
> > Signed-off-by: Ashok Raj <ashok.raj@...el.com>
> > [Haozhong: macro KVM_MCE_CAP_SUPPORTED => variable kvm_mce_cap_supported
> >            Only enable LMCE on Intel platform
> > 	   Check MSR_IA32_FEATURE_CONTROL when handling guest
> > 	     access to MSR_IA32_MCG_EXT_CTL]
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com>
[...]
> > @@ -6433,6 +6455,8 @@ static __init int hardware_setup(void)
> >  
> >  	kvm_set_posted_intr_wakeup_handler(wakeup_handler);
> >  
> > +	kvm_mce_cap_supported |= MCG_LMCE_P;
> 
> Ah, so virtual LMCE is available on all processors!  This is
> interesting, but it also makes it more complicated to handle in QEMU; a
> new QEMU generally doesn't require a new kernel.
> 
> Eduardo, any ideas?

(CCing libvirt list)

As we shouldn't make machine-type changes introduce new host
requirements, it looks like we need to either add a new set of
CPU models (unreasonable), or expect management software to
explicitly enable LMCE after ensuring the host supports it.

Or we could wait for a reasonable time after the feature is
available in the kernel, and declare that QEMU as a whole
requires a newer kernel. But how much time would be reasonable
for that?

Long term, I believe we should think of a better solution. I
don't think it is reasonable to require new libvirt code to be
written for every single low-level feature that requires a newer
kernel or newer host hardware. Maybe new introspection interfaces
that would allow us to drop the "no new requirements on
machine-type changes" rule?

-- 
Eduardo