[Xen-devel] RE: [PATCH][1/4] Enable 1GB for Xen HVM host page

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

Huang2, Wei wrote:
> The performance gain depends on applications. We have seen 5%
> performance improvement for certain benchmarks. But some have no
> perf. gains over 2MB (no performance degradation either). That is why
> I add an option to turn it off in case people don't want 1GB.   
> 
> I didn't have access to my guest QEMU until applying this patch from
> Dongxiao. Dongxiao might have other comments. 

When allocating guest memory without the patch, Xend will treat 3G-4G
as normal memory, ignoring the 3.75-4G MMIO hole. This patch modifies
MMIO start address to be 1GB aligned and could fix the issue.

Indeed, this patch may have some influence to pure 32bit guest,
changing its max memory from 3.75G to 3G.

Anyway, we can change it to alloc 2M pages between 3G and 4G if you
think this particular patch is improper. 

Thanks!
Dongxiao


> 
> -Wei
> 
> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Tuesday, February 23, 2010 1:57 AM
> To: Huang2, Wei; 'xen-devel@xxxxxxxxxxxxxxxxxxx'; Xu, Dongxiao
> Subject: Re: [PATCH][1/4] Enable 1GB for Xen HVM host page
> 
> What's the performance gain from 1GB mappings in HAP tables like?
> What about 
> just the (possible) extra 1GB mapping from this particular patch? Is
> it 
> worth making guest-visible efforts like this (admittedly small) one?
> After 
> all, the probably most frequently accessed 1GB starting at address 0x0
> cannot be done with a 1GB mapping and we live with it.
> 
>  -- Keir
> 
> On 22/02/2010 17:17, "Wei Huang" <wei.huang2@xxxxxxx> wrote:
> 
>> To support 1GB host page, this patch changes the MMIO starting
>> address to 1GB boundary. 
>> 
>> Signed-off-by: Dongxiao Xu <dongxiao.xu@xxxxxxxxx>
>> Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
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