Re: [PATCH 1/3] x86/P2M: synchronize fast and slow paths of p2m_get_page_from_gfn()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.03.2025 15:53, Roger Pau Monné wrote:
> On Wed, Feb 26, 2025 at 12:52:27PM +0100, Jan Beulich wrote:
>> Handling of both grants and foreign pages was different between the two
>> paths.
>>
>> While permitting access to grants would be desirable, doing so would
>> require more involved handling; undo that for the time being. In
>> particular the page reference obtained would prevent the owning domain
>> from changing e.g. the page's type (after the grantee has released the
>> last reference of the grant). Instead perhaps another reference on the
>> grant would need obtaining. Which in turn would require determining
>> which grant that was.
>>
>> Foreign pages in any event need permitting on both paths.
>>
>> Introduce a helper function to be used on both paths, such that
>> respective checking differs in just the extra "to be unshared" condition
>> on the fast path.
>>
>> While there adjust the sanity check for foreign pages: Don't leak the
>> reference on release builds when on a debug build the assertion would
>> have triggered. (Thanks to Roger for the suggestion.)
>>
>> Fixes: 80ea7af17269 ("x86/mm: Introduce get_page_from_gfn()")
>> Fixes: 50fe6e737059 ("pvh dom0: add and remove foreign pages")
>> Fixes: cbbca7be4aaa ("x86/p2m: make p2m_get_page_from_gfn() handle grant 
>> case correctly")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Just a couple of nits below (with a reply to your RFC).
> 
>> ---
>> RFC: While the helper could take const struct domain * as first
>>      parameter, for a P2M function it seemed more natural to have it
>>      take const struct p2m_domain *.
>>
>> --- a/xen/arch/x86/mm/p2m.c
>> +++ b/xen/arch/x86/mm/p2m.c
>> @@ -328,12 +328,45 @@ void p2m_put_gfn(struct p2m_domain *p2m,
>>      gfn_unlock(p2m, gfn_x(gfn), 0);
>>  }
>>  
>> +static struct page_info *get_page_from_mfn_and_type(
>> +    const struct p2m_domain *p2m, mfn_t mfn, p2m_type_t t)
> 
> Re your RFC: since it's a static function, just used for
> p2m_get_page_from_gfn(), I would consider passing a domain instead of
> a p2m_domain, as the solely usage of p2m is to obtain the domain.

Okay, will do.

>> +{
>> +    struct page_info *page;
>> +
>> +    if ( !mfn_valid(mfn) )
>> +        return NULL;
>> +
>> +    page = mfn_to_page(mfn);
>> +
>> +    if ( p2m_is_ram(t) )
> 
> Should this be a likely() to speed up the common successful path?

Well. Andrew's general advice looks to be to avoid likely() / unlikely()
in almost all situations. Therefore unless he positively indicates that
in a case like this using likely() is acceptable, I'd rather stay away
from adding that.

docs/process/coding-best-practices.pandoc could certainly do with some
rough guidelines on when adding these two is acceptable (or even
desirable). Right now to me it is largely unclear when their use is
deemed okay; it certainly doesn't match anymore what I was told some
20 years ago when I started working on Linux.