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Re: A question about the translation granule size supported by the vSMMU

From: Auger Eric
Subject: Re: A question about the translation granule size supported by the vSMMU
Date: Tue, 6 Apr 2021 21:50:11 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

Hi Kunkun,

On 3/27/21 3:24 AM, Kunkun Jiang wrote:
> Hi all,
> Recently, I did some tests on SMMU nested mode. Here is
> a question about the translation granule size supported by
> vSMMU.
> There is such a code in SMMUv3_init_regs():
>>    /* 4K and 64K granule support */
>>     s->idr[5] = FIELD_DP32(s->idr[5], IDR5, GRAN4K, 1);
>>     s->idr[5] = FIELD_DP32(s->idr[5], IDR5, GRAN64K, 1);
>>     s->idr[5] = FIELD_DP32(s->idr[5], IDR5, OAS, SMMU_IDR5_OAS); /* 44
>> bits */
> Why is the 16K granule not supported? I modified the code
> to support it and did not encounter any problems in the
> test. Although 4K and 64K minimal granules are "strongly
> recommended", I think vSMMU should still support 16K.😉
> Are there other reasons why 16K is not supported here?
no there aren't any. The main reasons were 16KB support is optional and
supporting it increases the test matrix. Also it seems quite a few
machines I have access to do support 16KB granule. On the others I get

"EFI stub: ERROR: This 16 KB granular kernel is not supported by your CPU".

Nevertheless I am not opposed to support it as it seems to work without
trouble. Just need to have an extra look at implied validity checks but
there shouldn't be much.


> When in SMMU nested mode, it may get errors if pSMMU
> doesn't support 16K but vSMMU supports 16K. But we
> can get some settings of pSMMU to avoid this situation.
> I found some discussions between Eric and Linu about
> this [1], but this idea does not seem to be implemented.
> [1] https://lists.gnu.org/archive/html/qemu-arm/2017-09/msg00149.html
> Best regards,
> Kunkun Jiang

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