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Re: [Xen-devel] use tasklet to handle init/sipi?

Keir Fraser wrote on 2013-03-25:
> On 25/03/2013 06:55, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> wrote:
>> Keir Fraser wrote on 2013-03-25:
>>> There are deadlock issues around directly locking and resetting a remote
>>> vcpu (e.g., buggy/malicious guest vcpu A sends INIT to vcpu B, and B does
>>> same to A).
>> Can you elaborate it? Does the lock impact hypervisor or just guest?
> INIT-handling path takes the domain lock. If two vcpus in same guest try to
> INIT each other, one will take the lock and then try to vcpu_pause() the
> other. But this will spin forever while that other vcpu itself waits to take
> the domain_lock.
> This seemed to me a fairly fundamental problem of vcpus directly resetting
> each other. Hence the deferral to tasklet context.

I see your point. But seems two vcpus call vcpu_pause() simultaneously without 
hold any lock also will cause the deadlock, see following code:
void vcpu_sleep_sync(struct vcpu *v)

    while ( !vcpu_runnable(v) && v->is_running )  // two vcpus arrived here at 
same time and waiting each vcpu will cause deadlock?


Also, should we care about such malicious guest? If the guest really did such 
thing, it just block himself. It just eat the cpu time which belong to himself. 
A malicious guest can run a non-stop loop to do same thing.

>  -- Keir
>>>  -- Keir
>>> On 25/03/2013 05:31, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> wrote:
>>>> Hi, Keir,
>>>> I am looking into a issue and found cs:17457 changes to use tasklet to
>>>> handle
>>>> init and sipi. And the comments only said "clean up". I wonder is there any
>>>> special reason to use tasklet to handle it? If no, I will send a patch to
>>>> call
>>>> handler directly instead via tasklet.
>>>> The background is that with APICv, it assume all apic write is succeed and
>>>> don't care the return value of vlapic_reg_write(). But the above logic need
>>>> the caller to check return value. This obviously will break APICv.
>>>> # HG changeset patch
>>>> # User Keir Fraser <keir.fraser@xxxxxxxxxx>
>>>> # Date 1208270873 -3600
>>>> # Node ID e15be54059e4bde8f5916269dedff5fc3812686a
>>>> # Parent  6691ae150d104127c097fd9f3a6acccc5ce43c52
>>>> x86, hvm: Clean up handling of APIC INIT and SIPI messages.
>>>> Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
>>>> best regards
>>>> yang
>> Best regards,
>> Yang

Best regards,

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