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Subject: Re: [PATCH 5.19 319/365] swiotlb: panic if nslabs is too small
To:     Yu Zhao <yuzhao@google.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>,
        stable <stable@vger.kernel.org>, Sasha Levin <sashal@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        iommu@lists.linux.dev
References: <20220823080118.128342613@linuxfoundation.org>
 <20220823080131.532813281@linuxfoundation.org>
 <c49d3b2b-9f5a-4257-9085-f7ac107cff40@oracle.com>
 <CAOUHufagA1x4jyjH9Q0RX65fwF3SyYHUTkNnB0S_t-2GqbiC2A@mail.gmail.com>
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <4d963d03-787f-992a-eb78-4719fb82cfae@oracle.com>
Date:   Tue, 23 Aug 2022 11:22:38 -0700
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On 8/23/22 10:51 AM, Yu Zhao wrote:
> On Tue, Aug 23, 2022 at 11:25 AM Dongli Zhang <dongli.zhang@oracle.com> wrote:
>>
>> Adding Robin, Yu and swiotlb list.
> 
> Thanks.
> 
>> There is an on-going discussion whether to revert this patch, because it breaks
>> a corner case in MIPS
> 
> I wouldn't call it a corner case. Cavium Octeon is the major platform
> we use to test Debian MIPS ports [1], and 4 out of 5 best-selling
> Wi-Fi routers are MIPS-based [2].
> 
> [1] https://urldefense.com/v3/__https://wiki.debian.org/MIPSPort__;!!ACWV5N9M2RV99hQ!NDVwU_XfPmfl_OSGxbroJXOjYmdrb-Vmbnx-zq0UIxkYNCetx4ZWdl6KlftLS9F5ORGT4t8F5YapMSSBiA$  
> [2] https://urldefense.com/v3/__https://www.amazon.com/bestsellers/pc/300189__;!!ACWV5N9M2RV99hQ!NDVwU_XfPmfl_OSGxbroJXOjYmdrb-Vmbnx-zq0UIxkYNCetx4ZWdl6KlftLS9F5ORGT4t8F5YacDz0Zlg$  
> 
>> when many kernel CONFIGs are not enabled (related to PCI
>> and device). As a result, MIPS pre-allocates only PAGE_SIZE buffer as swiotlb.
>>
>> https://urldefense.com/v3/__https://lore.kernel.org/all/20220820012031.1285979-1-yuzhao@google.com/__;!!ACWV5N9M2RV99hQ!NDVwU_XfPmfl_OSGxbroJXOjYmdrb-Vmbnx-zq0UIxkYNCetx4ZWdl6KlftLS9F5ORGT4t8F5YbWJyEn2A$  
>>
>> However, the core idea of the patch is to panic on purpose if the swiotlb is
>> configured with <1MB memory, in order to sync with the remap failure handler in
>> swiotlb_init_remap().
>>
>> Therefore, I am waiting for suggestion from Christoph whether (1) to revert this
>> patch, or (2) enforce the restriction to disallow <1MB allocation.
> 
> There are other archs (arm, ppc, riscv, s390, etc.) that call
> swiotlb_init(). Have you verified them all?
> 

The issue is not about swiotlb_init(). It is about swiotlb_adjust_size() where
the 'default_nslabs' is configured to a very small value (e.g., equivalent to
swiotlb=2). I do not see any arch can directly configure 'default_nslabs'.

There are only two callers (archs) of swiotlb_adjust_size(): amd/sev and
mips/cavium-octeon.

About amd/sev, it uses at least IO_TLB_DEFAULT_SIZE so that there is not any issue.

244         size = total_mem * 6 / 100;
245         size = clamp_val(size, IO_TLB_DEFAULT_SIZE, SZ_1G);
246         swiotlb_adjust_size(size);

In this case, only swiotlb=2 is allocated if PAGE_SIZE is 4K.

Thank you very much!

Dongli Zhang