Re: [Xen-devel] [PATCH] xen/arm: p2m: Prevent deadlock when using memaccess

[Julien Grall <julien.grall@xxxxxxx>]

On 06/03/18 11:06, Sergej Proskurin wrote:
> Hi Julien,
Hi Sergej,

> On 02/28/2018 04:25 PM, Julien Grall wrote:
> > Commit 7d623b358a4 "arm/mem_access: Add long-descriptor based gpt"
> > assumed the read-write lock can be taken recursively. However, this
> > assumption is wrong and will lead to deadlock when the lock is
> > contended.
> >
> > To avoid the nested lock, rework the locking in get_page_from_gva and
> > p2m_mem_access_check_and_get_page. The latter will now be called without
> > the p2m lock. The new locking in p2m_mem_accces_check_and_get_page will
> > not cover the translation of the VA to an IPA.
> >
> > This is fine because we can't promise that the stage-1 page-table have
> > changed behind our back (they are under guest control). Modification in
> > the stage-2 page-table can now happen, but I can't issue any potential
> > issue here except with the break-before-make sequence used when updating
> > page-table. gva_to_ipa may fail if the sequence is executed at the same
> > on another CPU. In that case we would fallback in the software lookup
> > path.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> The patch looks good to me. However, I did not understand the following
> sentence in the commit message: "... but I can't issue any potential
> issue here except with the break-before-make sequence used when updating
> page-table". What issue did you mean? Thank you.
To avoid breaking the coherency when updating the page-table the Arm Arm 
recommends to follow a break-before-make sequence (see G4-4916 in ARM 
DDI 0487C.a). It a 4 steps approach:
1. Replace the old entry with an invalid entry
2. Invalidate the cached old entries with a broadcasting TLB 
invalidation instruction
3. Wait for the completion of the TLB instruction with a dsb followed by 
an isb
4. Write the new entry

The P2M code is using this sequence, so an address translation walk 
using hardware instruction may fail if another CPU happen to update (e.g 
breaking a superpage) the P2M entry at the same time/ This is because 
there are a small window where the entry does not exist.
If you are interested in more details, you can look at the talk I gave a 
couple of years ago (see [1]).
> Reviewed-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
Thank you.

Cheers,

[1] https://schd.ws/hosted_files/xensummit2016/49/slides.pdf

--
Julien Grall

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