Re: [Xen-devel] [PATCH v5 2/4] x86/mem_sharing: copy a page_lock version to be internal to memshr

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Fri, May 17, 2019 at 1:21 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> >>> On 16.05.19 at 23:37, <tamas@xxxxxxxxxxxxx> wrote:
> > --- a/xen/include/asm-x86/mm.h
> > +++ b/xen/include/asm-x86/mm.h
> > @@ -356,24 +356,15 @@ struct platform_bad_page {
> >  const struct platform_bad_page *get_platform_badpages(unsigned int 
> > *array_size);
> >
> >  /* Per page locks:
> > - * page_lock() is used for two purposes: pte serialization, and memory 
> > sharing.
> > + * page_lock() is used for pte serialization.
> >   *
> >   * All users of page lock for pte serialization live in mm.c, use it
> >   * to lock a page table page during pte updates, do not take other locks 
> > within
> >   * the critical section delimited by page_lock/unlock, and perform no
> >   * nesting.
> >   *
> > - * All users of page lock for memory sharing live in mm/mem_sharing.c. 
> > Page_lock
> > - * is used in memory sharing to protect addition (share) and removal 
> > (unshare)
> > - * of (gfn,domain) tupples to a list of gfn's that the shared page is 
> > currently
> > - * backing. Nesting may happen when sharing (and locking) two pages -- 
> > deadlock
> > - * is avoided by locking pages in increasing order.
> > - * All memory sharing code paths take the p2m lock of the affected gfn 
> > before
> > - * taking the lock for the underlying page. We enforce ordering between 
> > page_lock
> > - * and p2m_lock using an mm-locks.h construct.
> > - *
> > - * These two users (pte serialization and memory sharing) do not collide, 
> > since
> > - * sharing is only supported for hvm guests, which do not perform pv pte 
> > updates.
> > + * The use of PGT_locked in mem_sharing does not collide, since 
> > mem_sharing is
> > + * only supported for hvm guests, which do not perform pv pte updates.
>
> Hmm, I thought we had agreed on you also correcting the wording of
> the sentence you now retain (as requested). As said before, a HVM
> (PVH to be precise) Dom0 can very well perform PV PTE updates, just
> not on itself. I had suggested the wording "which do not have PV PTEs
> updated" - I'd be fine for this to be folded in while committing, to avoid
> another round trip. With this

Thanks, I do seem to have missed that.

Tamas

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