Re: [Xen-devel] [PATCH v2 7/9] xen/arm: vgic: Optimize the way to store the target vCPU in the rank

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 06/10/15 14:59, Ian Campbell wrote:
> On Mon, 2015-10-05 at 13:51 +0100, Julien Grall wrote:
>> Xen is currently directly storing the value of GICD_ITARGETSR register
>> (for GICv2) and GICD_IROUTER (for GICv3) in the rank. This makes the
>> emulation of the registers access very simple but makes the code to get
>> the target vCPU for a given vIRQ more complex.
>>
>> While the target vCPU of an vIRQ is retrieved every time an vIRQ is
>> injected to the guest, the access to the register occurs less often.
>>
>> So the data structure should be optimized for the most common case
>> rather than the inverse.
>>
>> This patch introduces the usage of an array to store the target vCPU for
>> every interrupt in the rank. This will make the code to get the target
>> very quick. The emulation code will now have to generate the
>> GICD_ITARGETSR
>> and GICD_IROUTER register for read access and split it to store in a
>> convenient way.
>>
>> With the new way to store the target vCPU, the structure vgic_irq_rank
>> is shrinked down from 320 bytes to 92 bytes. This is saving about 228
> 
> "shrunk"
> 
>> bytes of memory allocated separately per vCPU.
>>
>> Note that with these changes, any read to those registers will list only
>> the target vCPU used by Xen. This is fine because the GIC spec doesn't
>> require to return exactly the value written and it can be seen as if we
>> decide to implement the register read-only.
> 
> I'd prefer if instead of "this is fine..." this said something like "We
> think this is likely to be OK because...(stuff)...".

Will do. I will resend the whole series fixing the different error you
mentioned within the series and add your various acked.

Regards,


-- 
Julien Grall

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