From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH 2/2] x86/hvm: Don't intercept #UD exceptions
	in general
Date: Wed, 27 Jan 2016 19:18:17 +0000
Message-ID: <56A917F9.4030003@citrix.com>
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To: Boris Ostrovsky <boris.ostrovsky@oracle.com>, Xen-devel <xen-devel@lists.xen.org>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>, Kevin Tian <kevin.tian@intel.com>, Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>, Jun Nakajima <jun.nakajima@intel.com>, Jan Beulich <JBeulich@suse.com>
List-Id: xen-devel@lists.xenproject.org

On 27/01/16 19:14, Boris Ostrovsky wrote:
> On 01/27/2016 01:59 PM, Andrew Cooper wrote:
>> On 27/01/16 18:49, Boris Ostrovsky wrote:
>>> On 01/27/2016 01:11 PM, Andrew Cooper wrote:
>>>> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
>>>> index 1d71216..1084e82 100644
>>>> --- a/xen/arch/x86/domctl.c
>>>> +++ b/xen/arch/x86/domctl.c
>>>> @@ -65,8 +65,20 @@ static void update_domain_cpuid_info(struct domain
>>>> *d,
>>>>                    .ecx = ctl->ecx
>>>>                }
>>>>            };
>>>> +        int old_vendor = d->arch.x86_vendor;
>>>>              d->arch.x86_vendor = get_cpu_vendor(vendor_id.str,
>>>> gcv_guest);
>>>> +
>>>> +        if ( is_hvm_domain(d) && (d->arch.x86_vendor != old_vendor) )
>>>> +        {
>>>> +            struct vcpu *v;
>>>> +
>>>> +            domain_pause(d);
>>>> +            for_each_vcpu( d, v )
>>>> +                hvm_update_guest_vendor(v);
>>>> +            domain_unpause(d);
>>>> +        }
>>>> +
>>>>            break;
>>>>        }
>>> Not specific to this patch, but shouldn't we pause/unpause domain for
>>> the whole routine?
>> Not specifically, although that might be better lonterm.
>>
>> In practice, this hypercall is only made as part of domain construction,
>> and never at domain runtime.
>
> Is it safe to unpause a domain here if it is not running?

Yes - all pausing/unpausing is reference counted, including the initial
systemcontroller pause reference taken (on behalf of the toolstack
domain) during the createdomain hypercall.

~Andrew