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Wed, 18 Dec 2024 23:12:20 -0800 (PST) Message-ID: <781a2f80-9ca6-4875-9b4a-ecef7694ae2e@suse.com> Date: Thu, 19 Dec 2024 08:12:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/xen/mmu: Increase MAX_CONTIG_ORDER To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Cc: sstabellini@kernel.org, oleksandr_tyshchenko@epam.com, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Thierry Escande References: <20241204171346.458105-1-thierry.escande@vates.tech> <8fb77778-b821-4e38-a835-54883ba14e4b@suse.com> <733e95a6-dd33-422a-a25b-9f08cef5860e@suse.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <733e95a6-dd33-422a-a25b-9f08cef5860e@suse.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 18.12.2024 12:24, Jürgen Groß wrote: > On 18.12.24 12:11, Thierry Escande wrote: >> >> >> On 12/12/2024 12:09, Juergen Gross wrote: >>> On 12.12.24 11:22, Jan Beulich wrote: >>>> On 11.12.2024 19:20, Thierry Escande wrote: >>>>> Hi Jan, >>>>> >>>>> On 09/12/2024 11:04, Jan Beulich wrote: >>>>>> On 04.12.2024 18:14, Thierry Escande wrote: >>>>>>> With change 9f40ec84a797 (xen/swiotlb: add alignment check for dma >>>>>>> buffers), the driver mpt3sas fails to load because it cannot allocate >>>>>>> its DMA pool for an allocation size of ~2,3 MBytes. This is because >>>>>>> the >>>>>>> alignement check added by 9f40ec84a797 fails and >>>>>>> xen_swiotlb_alloc_coherent() ends up calling >>>>>>> xen_create_contiguous_region() with a size order of 10 which is too >>>>>>> high >>>>>>> for the current max value. >>>>>>> >>>>>>> This patch increases the MAX_CONTIG_ORDER from 9 to 10 (4MB) to allow >>>>>>> such allocations. >>>>>>> >>>>>>> Signed-off-by: Thierry Escande >>>>>>> --- >>>>>>>   arch/x86/xen/mmu_pv.c | 2 +- >>>>>>>   1 file changed, 1 insertion(+), 1 deletion(-) >>>>>>> >>>>>>> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c >>>>>>> index 55a4996d0c04..7f110740e1a2 100644 >>>>>>> --- a/arch/x86/xen/mmu_pv.c >>>>>>> +++ b/arch/x86/xen/mmu_pv.c >>>>>>> @@ -2200,7 +2200,7 @@ void __init xen_init_mmu_ops(void) >>>>>>>   } >>>>>>>     /* Protected by xen_reservation_lock. */ >>>>>>> -#define MAX_CONTIG_ORDER 9 /* 2MB */ >>>>>>> +#define MAX_CONTIG_ORDER 10 /* 4MB */ >>>>>>>   static unsigned long discontig_frames[1<>>>>> >>>>>> While lacking respective commentary, bumping this value imo also >>>>>> needs to >>>>>> take into account Xen itself, at least commit-message-wise. The >>>>>> bumping is >>>>>> fine for Dom0 in any event. It is also fine for DomU-s with the >>>>>> defaults >>>>>> built into the hypervisor (orders 12 and 10 respectively for x86 and >>>>>> Arm), >>>>>> yet especially for Arm (and in the future PPC and RISC-V) any further >>>>>> bumping would be less straightforward. >>>>> >>>>> Thanks for pointing this out. On the Xen side, CONFIG_CTLDOM_MAX_ORDER >>>>> and CONFIG_HWDOM_MAX_ORDER seem big enough on all architectures. But I >>>>> see CONFIG_DOMU_MAX_ORDER set to 9 (also all archs). Won't that be a >>>>> problem for drivers trying to allocate more than that from a domU ? >>>> >>>> A driver assumes a (physical) device to be in the DomU, at which point it >>>> is CONFIG_PTDOM_MAX_ORDER which applies (PT standing for pass-through). >>>> >>>>>> However - does the driver really need this big a contiguous chunk? It >>>>>> would seem far more desirable to me to break that up some, if possible. >>>>> >>>>> Since this works on bare metal I'm afraid the driver maintainer (mpt >>>>> fusion driver) will just tell me to fix Xen. >>>> >>>> Well. The bigger such allocations, the larger the risk that on systems >>>> that have been up for a while such allocations can't be fulfilled anymore >>>> even in the bare metal case. >>> >>> Yes. I don't think we should just work around this issue without having >>> even tried to get the driver fixed. In case they refuse to change it, we >>> can still increase MAX_CONTIG_ORDER. >> >> Thanks for the feedback. I'll try to have a look at the driver if I have >> time to do so. > > Another thought would be to change the generic DMA allocation to not require > alignment based on the rounded up size, but on the largest power-of-2 chunk > fitting into the requested size. > > I don't see why a 2.3 MB memory allocation would need to be 4 MB aligned. It > should be perfectly fine to align it to 2 MB only. Yet that wouldn't make a difference here, would it? We'd still need a 4M chunk of contiguous space, just with less alignment. Jan