Re: [RFC PATCH 1/2] KVM: PPC: Book3S HV: Make virtual processor area registration more robust

[Alexander Graf <agraf@suse.de>]

On 17.01.2012, at 12:31, Paul Mackerras wrote:

> On Tue, Jan 17, 2012 at 10:27:26AM +0100, Alexander Graf wrote:
>=20
>> The thing I was getting at was not the map during the lifetime, but
>> the map during registration. Currently we have:
>>=20
>> 1) Set VPA to x
>> 2) Assign feature y to VPA
>> 3) Use VPA
>>=20
>> 1 and 2 are the slow path, 3 occurs more frequently. So we want 3 to
>> be fast. 1 and 2 don't matter that much wrt performance.
>>=20
>> You are currently mapping the VPA at /, which gets you into this
>> map/unmap mess trying to free the previous mapping. If you moved the
>> map to step 2 and only stored the GPA at step 1, all map+unmap
>> operations except for final unmaps would be in one spot, so you
>> wouldn't need to construct this big complex state machine.
>=20
> That might simplify things - I'll try it and see.  The worry with
> doing the map/pin at 2 is that if anything goes wrong we no longer
> have the opportunity to return an error for the H_REGISTER_VPA call,
> so I'll have to at least do some checking in 1, leading to possibly
> more code overall.

Well, then map and unmap it in step 1 and map it in step 2 again. We're =
in the slow path so performance isn't critical. Readability and =
maintainability however are :)


Alex