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Sun, 12 Jan 2025 23:20:31 -0800 (PST) Message-ID: Date: Mon, 13 Jan 2025 08:20:32 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] xen/pci: do not register devices outside of PCI segment scope To: Bjorn Helgaas Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, Juergen Gross , Stefano Stabellini , Oleksandr Tyshchenko , Roger Pau Monne References: <20250110222129.GA317771@bhelgaas> 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: <20250110222129.GA317771@bhelgaas> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10.01.2025 23:21, Bjorn Helgaas wrote: > On Fri, Jan 10, 2025 at 03:01:48PM +0100, Roger Pau Monne wrote: >> The PCI segment value is limited to 16 bits, however there are buses like VMD >> that fake being part of the PCI topology by adding segment with a number >> outside the scope of the PCI firmware specification range (>= 0x10000). The >> MCFG ACPI Table "PCI Segment Group Number" field is defined as having a 16 bit >> width. >> >> Attempting to register or manage those devices with Xen would result in errors >> at best, or overlaps with existing devices living on the truncated equivalent >> segment values. > > The ACPI _SEG method (ACPI r6.5, sec 6.5.6) and the corresponding > value in the MCFG table (PCI Firmware r3.3, sec 4.1.2) are clearly > 16-bit values. > > But otherwise, the segment value is pretty much an arbitrary software > value, and the kernel works fine with the larger domain values from > vmd_find_free_domain(), so this isn't quite enough to explain what the > issue with Xen is. > > Does Xen truncate the domain to 16 bits or use it to look up something > in ACPI? One of the involved public interface structs starts like this: struct physdev_pci_device_add { /* IN */ uint16_t seg; uint8_t bus; uint8_t devfn; ... So yes, wider segment values would be truncated. Plus, even if they weren't, there would need to be coordination between Dom0 and Xen on which devices gets which segment number, since - as you say - the assignment in Linux is pretty much arbitrary. Jan