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Subject: Re: [PATCH] KVM: VMX: fix vmwrite to invalid VMCS
To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-kernel@vger.kernel.org
References: <1435931368-27730-1-git-send-email-rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org, stable@vger.kernel.org,
        Yang Zhang <yang.z.zhang@intel.com>, Liang Li <liang.z.li@intel.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5596A679.1050202@redhat.com>
Date: Fri, 3 Jul 2015 17:12:57 +0200
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On 03/07/2015 15:49, Radim Krčmář wrote:
> fpu_activate is called outside of vcpu_load(), which means it should not
> touch VMCS, but fpu_activate needs to.  Avoid the call by moving it to a
> point where we know that the guest needs eager FPU and VMCS is loaded.
> 
> This will get rid of the following trace
> 
>  vmwrite error: reg 6800 value 0 (err 1)
>   [<ffffffff8162035b>] dump_stack+0x19/0x1b
>   [<ffffffffa046c701>] vmwrite_error+0x2c/0x2e [kvm_intel]
>   [<ffffffffa045f26f>] vmcs_writel+0x1f/0x30 [kvm_intel]
>   [<ffffffffa04617e5>] vmx_fpu_activate.part.61+0x45/0xb0 [kvm_intel]
>   [<ffffffffa0461865>] vmx_fpu_activate+0x15/0x20 [kvm_intel]
>   [<ffffffffa0560b91>] kvm_arch_vcpu_create+0x51/0x70 [kvm]
>   [<ffffffffa0548011>] kvm_vm_ioctl+0x1c1/0x760 [kvm]
>   [<ffffffff8118b55a>] ? handle_mm_fault+0x49a/0xec0
>   [<ffffffff811e47d5>] do_vfs_ioctl+0x2e5/0x4c0
>   [<ffffffff8127abbe>] ? file_has_perm+0xae/0xc0
>   [<ffffffff811e4a51>] SyS_ioctl+0xa1/0xc0
>   [<ffffffff81630949>] system_call_fastpath+0x16/0x1b
> 
> (Note: we also unconditionally activate FPU in vmx_vcpu_reset(), so the
>  removed code added nothing.)
> 
> Fixes: c447e76b4cab ("kvm/fpu: Enable eager restore kvm FPU for MPX")
> Cc: <stable@vger.kernel.org>
> Reported-by: Vlastimil Holer <vlastimil.holer@gmail.com>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Andrey reported offlist that the bug went away by reverting 1cde293.  So
the patch would at least need a new commit message. :)

I'm putting it on hold.

Paolo
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