Re: [PATCH 3/7] xen/p2m: put reference for superpage

[Julien Grall <julien@xxxxxxx>]

Hi,

CC-ing Roger as he is working on adding support for the foreign mapping 
on x86. Although, I am not expecting any implication as only 4KB mapping 
should be supported.
On 08/05/2024 22:05, Julien Grall wrote:
> On 07/05/2024 14:30, Luca Fancellu wrote:
> > > On 7 May 2024, at 14:20, Julien Grall <julien@xxxxxxx> wrote:
> > >
> > > Hi Luca,
> > >
> > > On 23/04/2024 09:25, Luca Fancellu wrote:
> > > > From: Penny Zheng <Penny.Zheng@xxxxxxx>
> > > > But today, p2m_put_l3_page could not handle superpages.
> > > This was done on purpose. Xen is not preemptible and therefore we 
> > > need to be cautious how much work is done within the p2m code.
> > > With the below proposal, for 1GB mapping, we may end up to call 
> > > put_page() up to 512 * 512 = 262144 times. put_page() can free 
> > > memory. This could be a very long operation.
> > > Have you benchmark how long it would take?
> > I did not, since its purpose was unclear to me and was not commented 
> > in the last serie from Penny.
> Honestly, I can't remember why it wasn't commented.
I skimmed through the code to check what we currently do for preemption.

{decrease, increase}_reservation() will allow to handle max_order() 
mapping at the time. On a default configuration, the max would be 4MB.
relinquish_p2m_mapping() is preempting every 512 iterations. One 
iteration is either a 4KB/2MB/1GB mapping.
relinquish_memory() is checking for preemption after every page.

So I think, it would be ok to allow 2MB mapping for static shared memory 
but not 1GB. relinquish_p2m_mapping() would also needs to be updated to 
take into account the larger foreign mapping.
I would consider to check for preemption if 't' is p2m_map_foreign and 
the order is above 9 (i.e. 2MB).
Cheers,

--
Julien Grall