Re: [Xen-devel] regression from c/s 22071:c5aed2e049bc (ept: Put locks around ept_get_entry) ?

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

Try this one.

FYI, the logic is pretty simple: Without this patch, ept_next_level()
gets a pointer and the logic goes through and reads the actual entry
piecemeal.  ept_set_entry() gets a pointer and goes through setting
bits in the actual entry piecemeal as well.  The idea is, have
ept_next_level() read the entire entry into a local variable, and then
act on that; and have ept_set_entry() write the new entry into a local
variable and then write the whole entry once.  So it's mostly changing
"ept_entry->" to "new_entry."

 -George

On Tue, Dec 14, 2010 at 12:37 PM, Jan Beulich <JBeulich@xxxxxxxxxx> wrote:
>>>> On 14.12.10 at 11:47, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
>> Yes, I have to apologize: I have a queue of PoD, p2m, and ept-related
>> fixes that I haven't pushed to the list because:
>> * they require non-negligible reworking
>> * it's been really difficult for me to set up an OSS-based system to test
>> them
>>
>> It actually turns out that doing locking in ept_get_entry() is the
>> wrong thing to do anyway; it can cause the following deadlock:
>>
>> p2m_change_type [grabs p2m lock] -> set_p2m_entry -> ept_set_entry ->
>> ept_set_middle_level -> p2m_alloc [grabs hap lock]
>>
>> write cr4 -> hap_update_paging_modes [grabes hap lock] ->
>> hap_update_cr3 -> gfn_to_mfn -> ept_get_entry -> [grabs p2m lock]
>>
>> Attached is a ported patch that removes locking in ept_get_entry(),
>> and implements access-once semantics for reading and writing.  This
>> solves the original problem (a race between reading and writing the
>> table) without causing deadlocks.  I haven't had a chance to test it
>> -- can you give it a spin?
>
> For really giving this a try I'd have to use it on 4.0, where it
> doesn't apply at all. Resolving the rejects is non-obvious for me
> in some cases, as I don't know this code well enough. Hence
> for the moment we'll just drop the bad backport of your first
> attempt.
>
> Jan
>
>
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Attachment: ept-access-once.4.0.diff
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