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

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

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)
{
    vcpu_sleep_nosync(v);

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

    sync_vcpu_execstate(v);
} 

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,
Yang



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel