Re: [Qemu-devel] [PATCH v3 0/4] target-ppc: Add FWNMI support in qemu for powerKVM guests

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 19, 2014 at 01:22:01PM +0100, Alexander Graf wrote:
> 
> 
> 
> > Am 19.11.2014 um 12:44 schrieb David Gibson <david@gibson.dropbear.id.au>:
> > 
> >> On Wed, Nov 19, 2014 at 11:32:56AM +0100, Alexander Graf wrote:
> >> 
> >> 
> >> 
> >>> Am 19.11.2014 um 06:48 schrieb Aravinda Prasad <aravinda@linux.vnet.ibm.com>:
> >>> 
> >>> 
> >>> 
> >>> On Tuesday 11 November 2014 08:54 AM, David Gibson wrote:
> >>> 
> >>> [..]
> >>> 
> >>>> 
> >>>> So, this may not still be possible depending on whether the KVM side
> >>>> of this is already merged, but it occurs to me that there's a simpler
> >>>> way.
> >>>> 
> >>>> Rather than mucking about with having to update the hypervisor on the
> >>>> RTAS location, they have qemu copy the code out of RTAS, patch it and
> >>>> copy it back into the vector, you could instead do this:
> >>>> 
> >>>> 1. Make KVM instead of immediately delivering a 0x200 for a guest
> >>>> machine check, cause a special exit to qemu.
> >>>> 
> >>>> 2. Have the register-nmi RTAS call store the guest side MC handler
> >>>> address in the spapr structure, but perform no actual guest code
> >>>> patching.
> >>>> 
> >>>> 3. Allocate the error log buffer independently from the RTAS blob,
> >>>> so qemu always knows where it is.
> >>>> 
> >>>> 4. When qemu gets the MC exit condition, instead of going via a
> >>>> patched 0x200 vector, just directly set the guest register state and
> >>>> jump straight into the guest side MC handler.
> >>> 
> >>> Before I proceed further I would like to know what others think about
> >>> the approach proposed above (except for step 3 - as per PAPR the error
> >>> log buffer should be part of RTAS blob and hence we cannot have error
> >>> log buffer independent of RTAS blob).
> >>> 
> >>> Alex, Alexey, Ben: Any thoughts?
> >> 
> >> If in doubt, stick to PAPR please.
> > 
> > Apart from (3), which was a misunderstanding on my part, this doesn't
> > diverge from PAPR - it's just a question of how we're implementing the
> > PAPR behaviour.
> 
> Do we need a guest handler at all? Couldn't we make MCs a new exit
> type and handle it all straight from QEMU?

Well, PAPR allows the OS to register a handler, which existing guests
will expect to be able to do.  The registered handler expects various
information collated for it though, so it isn't a "raw" 0x200 vector.

IIUC, traditionally pHyp implemented this by patching the guests 0x200
vector to collate the necessary information then jump to the supplied
handler.

I'm suggesting that instead we indeed make a new exit type, have qemu
collate the information internally then jump directly back into the
guest registered handler.

I'm not sure if that's quite what you were suggesting, but I think we
have pretty close to the same idea here.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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