Re: [Xen-devel] [PATCH v7 4/5] ix86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 3/13/2017 7:24 PM, Jan Beulich wrote:
> > > > On 11.03.17 at 09:42, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > On 3/11/2017 12:03 AM, Jan Beulich wrote:
> > > But there's a wider understanding issue I'm having here: What is
> > > an "entry" here? Commonly I would assume this to refer to an
> > > individual (4k) page, but it looks like you really mean table entry,
> > > i.e. possibly representing a 2M or 1G page.
> > Well, it should be an entry pointing to a 4K page(only).
> > For p2m_ioreq_server, we shall not meet huge page. Because they are
> > changed from p2m_ram_rw pages
> > in set_mem_type() -> p2m_change_type_one(), which calls p2m_set_entry()
> > with PAGE_ORDER_4K specified.
> And recombination of large pages won't ever end up hitting these?
Well, by recombination I guess you refer to the POD pages? I do not 
think p2m_ioreq_server
pages will be combined now, which means we do not need to worry about 
recounting the
p2m_ioreq_server entries while a split happens.

And as to type change from p2m_ram_rw to p2m_ioreq_server, even if this 
is done on a large
page, p2m_change_type_one() will split the page and only mark one ept 
entry(which maps to
a 4K page) as p2m_ioreq_server(other 511 entries remains as p2m_ram_rw). 
So I still believe
counting p2m_ioreq_server entries here is correct.

Besides, if we look from XenGT requirement side, it is guest graphic 
page tables we are trying to
write-protect, which are 4K in size.

Thanks
Yu

> Jan
>
>
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