Re: [Xen-devel] [PATCH 3 of 3] RFC: xenpaging: use waitqueue in ept_get

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

Below:
> On Tue, Nov 22, Andres Lagar-Cavilla wrote:
>
>> Hi Olaf,
>>
>> thanks for posting these RFC patches, great work!
>>
>> I have several comments. Most importantly, I want to hash out the
>> interactions between these wait queues and the work I've been doing on
>> p2m
>> synchronization. I've been runnning Xen with synchronized (i.e. locking)
>> p2m lookups for a few weeks now with little/no trouble. You can refer to
>> a
>> patch I posted to the list previously, which I can resend. (I'm waiting
>> on
>> feedback on some previous patches I sent to keep pushing on this work)
>>
>> - I think the waitqueue should be part of struct arch_domain. It is an
>> x86
>> construct that applies only to the base level p2m of the domain, not
>> every
>> possible p2m in a nested setting.
>
> Good point, I will move it. On the other hand, its current location cant
> be the final solution. A wake_up would start all waiting vcpus, not just
> the ones waiting for a certain gfn (in case of paging). There needs to
> be a better way for selective wake_up.

As long as you wrap the wait queue go-to-sleep action in a while loop that
rechecks the sleep condition before exiting the loop, this should be fine.
It's a standard idiom. There is an argument against spurious wake-ups, but
imhois no biggie.

>
>> - The right place to wait is not ept->get_entry, imho, but rather the
>> implementation-independent caller (get_gfn_type_access). More p2m
>> implementations could be added in the future (however unlikely that may
>> be) that can also perform paging.
>
> The wait could probably be moved one level up, yes.
>
>> - With locking p2m lookups, one needs to be careful about in_atomic. I
>> haven't performed a full audit of all callers, but this is what I can
>> say:
>> 1. there are no code paths that perform recursive p2m lookups, *on
>> different gfns*, with p2m_query_t != p2m_query
>> 2. there are recursive lookups of different gfns, with p2m_query_t ==
>> p2m_query, most notably pod sweeps. These do not represent a problem
>> here,
>> since those paths will skip over gfns that are paged.
>>
>> Why is this important? Because we need to be in a position where a code
>> snippet such as
>>
>> get_gfn_type_access(gfn) {
>> retry:
>>    p2m_lock()
>>    mfn = p2m->get_entry(gfn, &p2mt)
>>    if (p2mt == paging) {
>>        p2m_unlock()
>>        sleep_on_waitqueue()
>>        goto retry;
>>    }
>>
>> works. This will get us a non-racy p2m that sends vcpu's to sleep
>> without
>> holding locks. Makes sense?
>
> Something like that can be done if needed, yes.
>
>> - What is the plan for grant operations for pv-on-hvm drivers? Those
>> will
>> enter get_gfn* holding the domain lock, and thus in an atomic context.
>
> Is that a new thing? So far my PVonHVM guests did not encounter that
> issue. I see do_grant_table_op() taking the domain_lock, but is this
> code path really entered from the guest or rather from dom0?
> __get_paged_frame() returns GNTST_eagain, and that needs to be handled
> by callers of do_grant_table_op. linux-2.6.18-xen.hg has code to retry
> the grant operation in that case.

I'm on your side here. I've seen posts in the list about putting dom0 into
an hvm container using ept, though.

Andres
>
> Olaf
>



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