Re: [Qemu-devel] outlined TLB lookup on x86

[Xin Tong]

On Sun, Dec 8, 2013 at 2:54 AM, Xin Tong <address@hidden> wrote:
>
>
>
> On Thu, Nov 28, 2013 at 8:12 AM, Lluís Vilanova <address@hidden> wrote:
>>
>> Xin Tong writes:
>>
>> > Hi LIuis
>> > we can probably generate vector intrinsics using the tcg, e.g. add
>> > support to
>> > tcg to emit vector instructions directly in code cache
>>
>> There was some discussion long ago about adding vector instructions to
>> TCG, but
>> I don't remember what was the conclusion.
>>
>> Also remember that using vector instructions will "emulate" a
>> low-associativity
>> TLB; don't know how much better than a 1-way TLB will that be, though.
>>
>>
>> > why would a larger TLB make some operations slower, the TLB is a
>> > direct-mapped
>> > hash and lookup should be O(1) there. In the cputlb, the CPU_TLB_SIZE is
>> > always
>> > used to index into the TLB, i.e. (X & (CPU_TLB_SIZE -1)).
>>
>> It would make TLB invalidations slower (e.g., see 'tlb_flush' in
>> "cputlb.c"). And right now QEMU performs full TLB invalidations more
>> frequently
>> than the equivalent HW needs to, although I suppose that should be
>> quantified
>> too.

I see QEMU executed ~1M instructions per context switch for
qemu-system-x86_64. Is this because of the fact that the periodical
time interval interrupt is delivered in real time while QEMU is
significantly slower than real hw ?

Xin

>>
> you are right LIuis. QEMU does context switch quite more often that real hw,
> this is probably primarily due to the fact that QEMU is magnitude slower
> than real hw.  I am wondering where timer is emulated in QEMU system-x86_64.
> I imagine the guest OS must program the timers to do interrupt for context
> switches.
>
> Another question, what happens when a vcpu is stuck in an infinite loop ?
> QEMU must need an timer interrupt somewhere as well ?
>
> Is my understanding correct ?
>
> Xin
>>
>>
>> Lluis
>>
>> --
>>  "And it's much the same thing with knowledge, for whenever you learn
>>  something new, the whole world becomes that much richer."
>>  -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
>>  Tollbooth
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>