linux-kernel - Re: [PATCH] KVM: x86: Work around buggy MPX platform

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Date:	Mon, 10 Mar 2014 09:53:57 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Liu, Jinsong" <jinsong.liu@...el.com>,
	"'kvm@...r.kernel.org'" <kvm@...r.kernel.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'qemu-devel@...gnu.org'" <qemu-devel@...gnu.org>
Subject: Re: [PATCH] KVM: x86: Work around buggy MPX platform

Il 10/03/2014 09:48, Liu, Jinsong ha scritto:
> From 5854070994c5002b3a37577165ed3e82f36f712d Mon Sep 17 00:00:00 2001
> From: Liu Jinsong <jinsong.liu@...el.com>
> Date: Sat, 8 Mar 2014 04:40:02 +0800
> Subject: [PATCH] KVM: x86: Work around buggy MPX platform
>
> Work around buggy MPX platform which support MSR_IA32_BNDCFGS
> but has issue at, say, VMX ucode.
>
> Signed-off-by: Liu Jinsong <jinsong.liu@...el.com>
> ---
>  arch/x86/kvm/x86.c |   14 +++++++++++++-
>  1 files changed, 13 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1e91a24..1fc184d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3936,6 +3936,16 @@ static void kvm_init_msr_list(void)
>  	for (i = j = KVM_SAVE_MSRS_BEGIN; i < ARRAY_SIZE(msrs_to_save); i++) {
>  		if (rdmsr_safe(msrs_to_save[i], &dummy[0], &dummy[1]) < 0)
>  			continue;
> +
> +		/*
> +		 * Work around some buggy MPX platform which support
> +		 * MSR_IA32_BNDCFGS but has issue at, say, VMX ucode.
> +		 */
> +		if ((msrs_to_save[i] == MSR_IA32_BNDCFGS) &&

Too many parentheses in this "if".


> +			(kvm_x86_ops->mpx_supported   ?
> +			!kvm_x86_ops->mpx_supported() : 1))

Better: "!kvm_x86_ops->mpx_supported || !kvm_x86_ops->mpx_supported()" 
but I'm leaning towards adding an implementation of mpx_supported to SVM 
too (it will always return false).

> +				continue;
> +
>  		if (j < i)
>  			msrs_to_save[j] = msrs_to_save[i];
>  		j++;
> @@ -5576,9 +5586,11 @@ int kvm_arch_init(void *opaque)
>  		goto out_free_percpu;
>
>  	kvm_set_mmio_spte_mask();
> -	kvm_init_msr_list();
>
>  	kvm_x86_ops = ops;
> +
> +	kvm_init_msr_list();
> +
>  	kvm_mmu_set_mask_ptes(PT_USER_MASK, PT_ACCESSED_MASK,
>  			PT_DIRTY_MASK, PT64_NX_MASK, 0);
>
>

Thanks Liu.  Indeed I could not reproduce Robert's issue, though it 
might be possible to get it with nested virtualization.  I'll send a 
similar patch that I was testing.

Paolo
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