Re: [Xen-devel] [PATCH v5] x86/p2m: use large pages for MMIO mappings

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.01.16 at 23:35, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Tuesday, January 26, 2016 12:19 AM
>> 
>> When mapping large BARs (e.g. the frame buffer of a graphics card) the
>> overhead of establishing such mappings using only 4k pages has,
>> particularly after the XSA-125 fix, become unacceptable. Alter the
>> XEN_DOMCTL_memory_mapping semantics once again, so that there's no
>> longer a fixed amount of guest frames that represents the upper limit
>> of what a single invocation can map. Instead bound execution time by
>> limiting the number of iterations (regardless of page size).
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx> for VMX part.
> 
> Curious. When you say "become unacceptable", how bad is it? mostly
> impact the boot time?

Yes, guest boot time. I don't have a reference to the original report
at hand, but that was what someone (Konrad?) had reported. I've
never seen the issue myself, largely because I've never made any
attempt at GPU pass-through.

Jan


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