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Message-ID: Date: Mon, 4 Dec 2023 09:24:53 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 4/5] [FUTURE] xen/arm: enable vPCI for domUs Content-Language: en-US To: Stefano Stabellini Cc: Stewart Hildebrand , xen-devel@lists.xenproject.org, Julien Grall , Bertrand Marquis , Michal Orzel , Volodymyr Babchuk , Andrew Cooper , George Dunlap , Wei Liu , Paul Durrant , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <20231113222118.825758-1-stewart.hildebrand@amd.com> <20231113222118.825758-5-stewart.hildebrand@amd.com> 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; 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>>> devfn = PCI_DEVFN(machine_sbdf); >>> >>> + if ( needs_vpci(d) && !has_vpci(d) ) >>> + { >>> + printk(XENLOG_G_WARNING "Cannot assign %pp to %pd: vPCI support not enabled\n", >>> + &PCI_SBDF(seg, bus, devfn), d); >>> + ret = -EPERM; >>> + break; >> >> I think this is likely too restrictive going forward. The current >> approach is indeed to enable vPCI on a per-domain basis because that's >> how PVH dom0 uses it, due to being unable to use ioreq servers. >> >> If we start to expose vPCI suport to guests the interface should be on >> a per-device basis, so that vPCI could be enabled for some devices, >> while others could still be handled by ioreq servers. >> >> We might want to add a new flag to xen_domctl_assign_device (used by >> XEN_DOMCTL_assign_device) in order to signal whether the device will >> use vPCI. > > Actually I don't think this is a good idea. I am all for flexibility but > supporting multiple different configurations comes at an extra cost for > both maintainers and contributors. I think we should try to reduce the > amount of configurations we support rather than increasing them > (especially on x86 where we have PV, PVH, HVM). > > I don't think we should enable IOREQ servers to handle PCI passthrough > for PVH guests and/or guests with vPCI. If the domain has vPCI, PCI > Passthrough can be handled by vPCI just fine. I think this should be a > good anti-feature to have (a goal to explicitly not add this feature) to > reduce complexity. Unless you see a specific usecase to add support for > it? I could see very special devices to want accompanying by a special ioreq server. I could also see tandem pass-through/emulated device pairs to potentially be of use and require coordination in the same ioreq server. That said, I wouldn't insist on allowing a mix of vPCI and ioreq servers to be used until an actual use case (with code supporting it) actually appears. Jan