[amdgpu] Is it possible to disable page tables and use the physical address directly in amdgpu

[amdgpu] Is it possible to disable page tables and use the physical address directly in amdgpu

[Smith John]

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Hello!
I was trying to figure out the impact of gpu page tables on applications'
performance. I noticed that there are 16 vmids supported by the hardware *Vega
10*. Is it possible to use physical address directly in some vmids, or use
physical address globally?

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Re: [amdgpu] Is it possible to disable page tables and use the physical address directly in amdgpu

[Christian König]

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Am 01.12.20 um 07:58 schrieb Smith John:
> Hello!
> I was trying to figure out the impact of gpu page tables on 
> applications' performance. I noticed that there are 16 vmids supported 
> by the hardware /Vega 10/. Is it possible to use physical address 
> directly in some vmids, or use physical address globally?

No. VMID0 is used by the kernel for jobs like copying things from A to B 
and even there we use the VM remapping functionality.

Regards,
Christian.

>
>
> _______________________________________________
> amd-gfx mailing list
> amd-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/amd-gfx


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Re: [amdgpu] Is it possible to disable page tables and use the physical address directly in amdgpu

[Smith John]

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 Hi Christian,
Thanks for your reply. I agree with you that the VMID0 is special and
remapping is important. I was not sure if different VIMDs could have
different settings, such as enable/disable page tables.
Or to put it another way, I was wondering if the hardware supports purely
physical addressing like the real mode in CPUs, or page tables are
essential for the hardware.
More specifically, assuming it supports "real mode", to copy things from A
to B, one could allocate rings which are accessible by MMIO and fill sdma
packets using physical address to transfer data.

Regards,
Smith

Christian König <ckoenig.leichtzumerken@gmail.com> 于2020年12月1日周二 下午5:50写道：

> Am 01.12.20 um 07:58 schrieb Smith John:
>
> Hello!
> I was trying to figure out the impact of gpu page tables on applications'
> performance. I noticed that there are 16 vmids supported by the hardware *Vega
> 10*. Is it possible to use physical address directly in some vmids, or
> use physical address globally?
>
>
> No. VMID0 is used by the kernel for jobs like copying things from A to B
> and even there we use the VM remapping functionality.
>
> Regards,
> Christian.
>
>
>
> _______________________________________________
> amd-gfx mailing listamd-gfx@lists.freedesktop.orghttps://lists.freedesktop.org/mailman/listinfo/amd-gfx
>
>
>

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Re: [amdgpu] Is it possible to disable page tables and use the physical address directly in amdgpu

[Christian König]

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The page tables can be enabled/disabled on a per VMID basis, but memory 
management in the core kernel works with pages.

So you need a relocation table just because of this. Additional to that 
the TLB is more than big enough, so there isn't much performance gain if 
you use huge pages.

Please note that the VM subsystem also supports giant pages, so if your 
application manages to allocate things in chunks of at least 1GB you 
only get a single page table entry for that.

Regards,
Christian.

Am 01.12.20 um 12:28 schrieb Smith John:
>  Hi Christian,
> Thanks for your reply. I agree with you that the VMID0 is special and 
> remapping is important. I was not sure if different VIMDs could have 
> different settings, such as enable/disable page tables.
> Or to put it another way, I was wondering if the hardware supports 
> purely physical addressing like the real mode in CPUs, or page tables 
> are essential for the hardware.
> More specifically, assuming it supports "real mode", to copy things 
> from A to B, one could allocate rings which are accessible by MMIO and 
> fill sdma packets using physical address to transfer data.
>
> Regards,
> Smith
>
> Christian König <ckoenig.leichtzumerken@gmail.com 
> <mailto:ckoenig.leichtzumerken@gmail.com>> 于2020年12月1日周二 
> 下午5:50写道：
>
>     Am 01.12.20 um 07:58 schrieb Smith John:
>>     Hello!
>>     I was trying to figure out the impact of gpu page tables on
>>     applications' performance. I noticed that there are 16 vmids
>>     supported by the hardware /Vega 10/. Is it possible to use
>>     physical address directly in some vmids, or use physical address
>>     globally?
>
>     No. VMID0 is used by the kernel for jobs like copying things from
>     A to B and even there we use the VM remapping functionality.
>
>     Regards,
>     Christian.
>
>>
>>
>>     _______________________________________________
>>     amd-gfx mailing list
>>     amd-gfx@lists.freedesktop.org  <mailto:amd-gfx@lists.freedesktop.org>
>>     https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>
>
> _______________________________________________
> amd-gfx mailing list
> amd-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/amd-gfx


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