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Thu, 29 Jan 2026 03:43:50 -0800 (PST) Message-ID: Date: Thu, 29 Jan 2026 12:43:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/4] PCI: determine whether a device has extended config space To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Julien Grall , Stefano Stabellini , Anthony PERARD , Michal Orzel , Stewart Hildebrand References: <58091dc1-7bda-4536-8200-2d0a5679d4d1@suse.com> <2d5e017f-1675-4753-9494-2b664c4ca7fa@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 29.01.2026 11:33, Roger Pau Monné wrote: > On Thu, Jan 29, 2026 at 09:15:30AM +0100, Jan Beulich wrote: >> On 28.01.2026 18:49, Roger Pau Monné wrote: >>> On Mon, Jan 19, 2026 at 03:46:55PM +0100, Jan Beulich wrote: >>>> Legacy PCI devices don't have any extended config space. Reading any part >>>> thereof may return all ones or other arbitrary data, e.g. in some cases >>>> base config space contents repeatedly. >>>> >>>> Logic follows Linux 6.19-rc's pci_cfg_space_size(), albeit leveraging our >>>> determination of device type; in particular some comments are taken >>>> verbatim from there. >>>> >>>> Signed-off-by: Jan Beulich > > Acked-by: Roger Pau Monné Thanks. There'll be a v3 though in any event, to add the command line option that you asked for. I'll take the liberty to retain the ack (whereas I've dropped Stewart's R-b). >>>> Linux also has CONFIG_PCI_QUIRKS, allowing to compile out the slightly >>>> risky code (as reads may in principle have side effects). Should we gain >>>> such, too? >>> >>> I would be fine with just a command line to disable the newly added >>> behavior in case it causes issues. >> >> Can do. Will need to get creative as to the name of such an option. > > pci=check-ext-cfg=? Kind of a mouthful. As we already have "pci=", I've added a "pci=no-quirks" sub-option there. >>>> @@ -718,6 +721,11 @@ int pci_add_device(u16 seg, u8 bus, u8 d >>>> >>>> list_add(&pdev->vf_list, &pf_pdev->vf_list); >>>> } >>>> + >>>> + if ( !pdev->ext_cfg ) >>>> + printk(XENLOG_WARNING >>>> + "%pp: VF without extended config space?\n", >>>> + &pdev->sbdf); >>> >>> You possibly also want to check that the PF (pf_pdev in this context I >>> think) also has ext_cfg == true. >> >> I don't think so. No extended config space on a PF means no PF in that sense >> in the first place, for then there not being any SR-IOV capability. > > Right, but won't it be possible for Xen to not be aware of the > ECAM region for that device, yet the hardware domain somehow managed > to enable SR-IOV it and request to register a VF? Then we're screwed elsewhere, when we try to read the SR-IOV capability ourselves. >>>> @@ -1041,6 +1049,75 @@ enum pdev_type pdev_type(u16 seg, u8 bus >>>> return pos ? DEV_TYPE_PCIe_ENDPOINT : DEV_TYPE_PCI; >>>> } >>>> >>>> +void pci_check_extcfg(struct pci_dev *pdev) >>>> +{ >>>> + unsigned int pos, sig; >>>> + >>>> + pdev->ext_cfg = false; >>> >>> I think I would prefer if the ext_cfg field is only modified once Xen >>> know the correct value to put there. >> >> Well, my main point of doing it this way is that the code ends up being a >> little easier to follow. Especially without the optimization talked about >> near the top, there inevitably will be a window in time where what the >> field says is wrong. With the optimization there'll be two main cases: >> - MCFG becoming newly available: The field starts out false in this case, >> i.e. the write above is a no-op. >> - MCFG disappearing (largely hypothetical, I think): The field may start >> out true in this case, but will go false unless we have another access >> mechanism for extended config space. It then can as well be set to >> false as early as possible. > > Yes, with the optimization to not re-parse existing MMCFGs there's no > transient windows where the filed is wrongly set. When there's no change. When there is a change, there'll still be such a window. Unavoidably, though. > I also think the registering of MMCFG ares with Xen should be done > ahead of the OS attempting to access the config space, and hence it's > not possible for there to be in-flight accesses that could see > transient invalid pdev->ext_cfg values. Yes, for Dom0 alone all should be fine. My worry here is with dom0less and Hyperlaunch. Jan