Re: [Xen-devel] [RFC PATCH 0/3] xen/privcmd: support for paged-out frames

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Aug 24, 2012, at 8:14 AM, Ian Campbell wrote:

> On Fri, 2012-08-24 at 12:58 +0100, David Vrabel wrote:
>> Is PRIVCMD_MMAPBATCH_V2 actually required?  It doesn't seem to do much
>> more than the V1 interface.  Perhaps the fix in patches #1 and #2
>> sufficient to make libxc work?
> 
> The V1 interface has some hideous misfeature wrt error reporting iirc.
> 
> It doesn't report the actual per-frame error code, just the "an error"
> signal through setting the top nibble (already a nasty interface!), IIRC
> 
> Aha: http://xenbits.xen.org/hg/linux-2.6.18-xen.hg/rev/6d6c3dd995c0
IIuc, David's approach in these patches was
hypervisor rc -> encode into mfn field -> decode into error rval if available.

The changeset Ian references shows a different approach is needed. Perhaps:
1. Provide local err array in all cases
2. Always store hypervisor rc into err array
3. if version 2, copy into user-space. If version 1, encode into user-space mfn

Andres

> 
> It was knackered on 64 bit (actually set bits 28-31...)...
> 
> Ian.
> 


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