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[37.24.206.209]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ab7f779d9aesm97924066b.63.2025.02.12.03.49.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 12 Feb 2025 03:49:08 -0800 (PST) Message-ID: <38eb09be-67fc-46ff-9e78-c37c30f50e4b@suse.com> Date: Wed, 12 Feb 2025 12:49:07 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] xen/swiotlb: don't destroy contiguous region in all cases To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Cc: Stefano Stabellini , Boris Ostrovsky , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Oleksandr Tyshchenko , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, x86@kernel.org, iommu@lists.linux.dev References: <20250211120432.29493-1-jgross@suse.com> <20250211120432.29493-3-jgross@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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 12.02.2025 12:11, Jürgen Groß wrote: > On 12.02.25 08:38, Jan Beulich wrote: >> On 11.02.2025 13:04, Juergen Gross wrote: >>> In case xen_swiotlb_alloc_coherent() needed to create a contiguous >>> region only for other reason than the memory not being compliant with >>> the device's DMA mask, there is no reason why this contiguous region >>> should be destroyed by xen_swiotlb_free_coherent() later. Destroying >>> this region should be done only, if the memory of the region was >>> allocated with more stringent placement requirements than the memory >>> it did replace. >> >> I'm not convinced of this: Even the mere property of being contiguous >> may already be enough to warrant freeing when possible. The hypervisor >> may not have that many contiguous areas available. The bigger the >> chunk, the more important to give it back once no longer needed in >> this shape. > > Really? When creating a domain Xen tries to use GB pages and 2MB pages as > much as possible. Why would this special case here have more restrictions? There aren't many Gb pages to be had from the space below 4Gb; frequently there'll be at most one (covering the range from 1 to 2 Gb). For that as well as 2Mb ones I think it is a mistake that Xen may hand them out, when the caller could fall back to 4k allocations. Thing is that without extra information it's hard to come up with a good heuristic to decide whether the caller is capable of falling back. Perhaps e.g. populate_physmap() should add MEMF_no_dma to higher order allocation requests targeting other than the current domain, or when !d->creation_finished. Jan