Re: [Xen-devel] [PATCH v2 1/2] hvm/vmx: save dr7 during vmx_vmcs_save

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.02.16 at 18:17, <tlengyel@xxxxxxxxxxx> wrote:
> On Mon, Feb 15, 2016 at 10:06 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >>> On 15.02.16 at 17:55, <tlengyel@xxxxxxxxxxx> wrote:
>> > All paths that end up using this dr7 value in vm_event have v==current,
>> so
>> > right now there is no caller to this function using dr7 where v!=current.
>> > Future callers where v!=current could do so indeed.
>>
>> Well, first of all the vm_event consumers of this data are secondary.
>> The primary consumer is the VM save logic, which runs with
>> v != current. It just so happens that hvm_save_cpu_ctxt() already
>> ignores that field and uses v->arch.debugreg[7] instead. Hence
>> we're back to square one: How much of an overhead is the extra
>> VMREAD (the data gathered by which the primary consumer has no
>> use for)?
> 
> 
> I don't have an answer to that but I don't think it's very significant. My
> original intention was to introduce a separate hvm function to do this
> saving of dr7 which was voted against by Andrew. I personally don't have a
> use for dr7 at the moment either way.

Andrew, thoughts? (As usual, a single such unnecessary addition
may not be noticable, but once we set a precedent, more might
follow, until we get into the noticable range.)

Jan


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