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Re: [Qemu-devel] Poking a sun4v machine

From: Artyom Tarasenko
Subject: Re: [Qemu-devel] Poking a sun4v machine
Date: Wed, 2 May 2012 16:38:16 +0200

On Tue, May 1, 2012 at 4:06 PM, Blue Swirl <address@hidden> wrote:
> On Tue, May 1, 2012 at 13:54, Artyom Tarasenko <address@hidden> wrote:
>> On Tue, May 1, 2012 at 11:25 AM, Blue Swirl <address@hidden> wrote:
>>> On Mon, Apr 30, 2012 at 17:38, Artyom Tarasenko <address@hidden> wrote:
>>>> On Mon, Apr 30, 2012 at 7:15 PM, Andreas Färber <address@hidden> wrote:
>>>>> Am 30.04.2012 18:39, schrieb Artyom Tarasenko:
>>>>>> Tried to boot QEMU Niagara machine with the firmware from the
>>>>>> OpenSPARC T1 emulator ( www.opensparc.net/opensparc-t1/download.html )
>>>>>> , and it dies very early.
>>>>>> The reason: in translate.c
>>>>>> #define hypervisor(dc) (dc->mem_idx == MMU_HYPV_IDX)
>>>>>> #define supervisor(dc) (dc->mem_idx >= MMU_KERNEL_IDX)
>>>>>> and the dc->mem_idx is initialized like this:
>>>>>>     if (env1->tl > 0) {
>>>>>>         return MMU_NUCLEUS_IDX;
>>>>>>     } else if (cpu_hypervisor_mode(env1)) {
>>>>>>         return MMU_HYPV_IDX;
>>>>>>     } else if (cpu_supervisor_mode(env1)) {
>>>>>>         return MMU_KERNEL_IDX;
>>>>>>     } else {
>>>>>>         return MMU_USER_IDX;
>>>>>>     }
>>>>>> Which seems to be conceptually incorrect. After reset tl == MAXTL, but
>>>>>> still super- and hyper-visor bits are set, so both supervisor(dc) and
>>>>>> hypervisor(dc) must return 1 which is impossible in the current
>>>>>> implementation.
>>>>>> What would be the proper way to fix it? Make mem_idx bitmap, add two
>>>>>> more variables to DisasContext, or ...?
>>>>>> Some other findings/questions:
>>>>>>     /* Sun4v generic Niagara machine */
>>>>>>     {
>>>>>>         .default_cpu_model = "Sun UltraSparc T1",
>>>>>>         .console_serial_base = 0xfff0c2c000ULL,
>>>>>> Where is this address coming from? The OpenSPARC Niagara machine has a
>>>>>> "dumb serial" at 0x1f10000000ULL.
>>>>>> And the biggest issue: UA2005 (as well as UA2007) describe a totally
>>>>>> different format for a MMU TTE entry than the one sun4u CPU are using.
>>>>>> I think the best way to handle it would be splitting off Niagara
>>>>>> machine, and #defining MMU bits differently for sun4u and sun4v
>>>>>> machines.
>>>>>> Do we the cases in qemu where more than two (qemu-system-xxx and
>>>>>> qemu-system-xxx64) binaries are produced?
>>>>>> Would the name qemu-system-sun4v fit the naming convention?
>>>>> We have such a case for ppc (ppcemb) and it is kind of a maintenance
>>>>> nightmare - I'm working towards getting rid of it with my QOM CPU work.
>>>>> Better avoid it for sparc in the first place.
>>>>> Instead, you should add a callback function pointer to SPARCCPUClass
>>>>> that you initialize based on CPU model so that is behaves differently at
>>>>> runtime rather than at compile time.
>>>>> Or if it's just about the class_init then after the Hard Freeze I can
>>>>> start polishing my subclasses for sparc so that you can add a special
>>>>> class_init for Niagara.
>>>> But this would mean that the defines from
>>>> #define TTE_NFO_BIT (1ULL << 60)
>>>> to
>>>> #define TTE_PGSIZE(tte)     (((tte) >> 61) & 3ULL)
>>>> inclusive would need to be replaced with functions and variables?
>>>> Sounds like a further performance regression for sun4u?
>>> There could be parallel definitions for sun4u (actually UltraSparc-III
>>> onwards the MMU is again different) and sun4v.
>>> At tlb_fill(), different implementations can be selected based on MMU
>>> model. For ASI accesses, we can add conditional code but for higher
>>> performance, some checks can be moved to translation time.
>> Can be done, but what is the gain of having it runtime configurable?
> I was thinking of code like this in:
> switch (env->mmu_model) {
> case MMU_US2:
>   return tlb_fill_us2(..);
> case MMU_US3:
>   return tlb_fill_us3(..);
> case MMU_US4:
>   return tlb_fill_us4(..);
> case MMU_T1:
>   return tlb_fill_t1(..);
> case MMU_T2:
>   return tlb_fill_t2(..);
> }
> The perfomance cost shouldn't be too high. Alternatively a function
> pointer could be set up.

Actually I was more worried about get_physical_address_* than filling,
there we would have to use variables instead of constants and
functions instead of macros.

> Yes, we can always provide the register bank, older models just access
> some of those.
>> cpu_change_pstate should probably have another parameter (new_GL)
>> which is only valid for sun4v.
>> And, depending on a trap type, env->htba has to be taken instead of
>> env->tbr. To me it looks like at the end do_interrupt will have less
>> common parts between sun4u and sun4v than specific ones.
> Same as tlb_fill(), switch() or function pointer. The functions are different.
> This is unavoidable (unless maybe in the future the TLB handling can
> be pushed partially higher so mmu_idx parameters can be eliminated)
> and the performance cost is not great.

So, altogether you'd still prefer run-time checks over having
qemu-system-sun4v (or -sparc64v) ?

Artyom Tarasenko

solaris/sparc under qemu blog: http://tyom.blogspot.com/search/label/qemu

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