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Mon, 03 Feb 2025 05:00:37 -0800 (PST) Message-ID: Date: Mon, 3 Feb 2025 14:00:36 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-4.20 2/3] x86/PCI: init segments earlier To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Oleksii Kurochko References: <2bb9d3c4-0761-4d63-8193-29293e35eb04@suse.com> <940ccd1b-9ad8-4b68-a035-36f45326872b@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 03.02.2025 13:45, Roger Pau Monné wrote: > On Thu, Jan 30, 2025 at 12:12:31PM +0100, Jan Beulich wrote: >> In order for amd_iommu_detect_one_acpi()'s call to pci_ro_device() to >> have permanent effect, pci_segments_init() needs to be called ahead of >> making it there. Without this we're losing segment 0's r/o map, and thus >> we're losing write-protection of the PCI devices representing IOMMUs. >> Which in turn means that half-way recent Linux Dom0 will, as it boots, >> turn off MSI on these devices, thus preventing any IOMMU events (faults >> in particular) from being reported on pre-x2APIC hardware. >> >> As the acpi_iommu_init() invocation was moved ahead of >> acpi_mmcfg_init()'s by the offending commit, move the call to >> pci_segments_init() accordingly. >> >> Fixes: 3950f2485bbc ("x86/x2APIC: defer probe until after IOMMU ACPI table parsing") >> Signed-off-by: Jan Beulich >> --- >> Of course it would have been quite a bit easier to notice this issue if >> radix_tree_insert() wouldn't work fine ahead of radix_tree_init() being >> invoked for a given radix tree, when the index inserted at is 0. >> >> While hunting down various other dead paths to actually find the root >> cause, it occurred to me that it's probably not a good idea to fully >> disallow config space writes for r/o devices: Dom0 won't be able to size >> their BARs (luckily the IOMMU "devices" don't have any, but e.g. serial >> ones generally will have at least one), for example. Without being able >> to size BARs it also will likely be unable to correctly account for the >> address space taken by these BARs. However, outside of vPCI it's not >> really clear to me how we could reasonably emulate such BAR sizing >> writes - we can't, after all, allow Dom0 to actually write to the >> underlying physical registers, yet we don't intercept reads (i.e. we >> can't mimic expected behavior then). > > For properly sizing the domain will also attempt to toggle the memory > decoding bit ahead of sizing the BARs, and letting that trough will > break the usage of the device from Xen. IOW: we would likely need to > emulate a fair amount of device state to make the view coherent from a > guest PoV, but is it worth it for a device that the hardware domain > cannot interact with? > > Would it make more sense to just hide those devices instead of > allowing read-only access to their PCI config space? No, I don't think so. The original reason is still valid: We want such devices to be enumerable by Dom0. Consider just this one implication from us not permitting that: What if such a device is part of a multi- function one, at func 0? Then we'd effectively hide all other devices at the same bus/dev, too. >> --- a/xen/arch/x86/x86_64/mmconfig-shared.c >> +++ b/xen/arch/x86/x86_64/mmconfig-shared.c >> @@ -402,8 +402,6 @@ void __init acpi_mmcfg_init(void) >> { >> bool valid = true; >> >> - pci_segments_init(); >> - >> /* MMCONFIG disabled */ >> if ((pci_probe & PCI_PROBE_MMCONF) == 0) >> return; >> --- a/xen/drivers/passthrough/x86/iommu.c >> +++ b/xen/drivers/passthrough/x86/iommu.c >> @@ -55,6 +55,8 @@ void __init acpi_iommu_init(void) >> { >> int ret = -ENODEV; >> >> + pci_segments_init(); > > My preference might be to just place the pci_segments_init() call in > __start_xen(), As said in reply to Andrew - I was considering doing so as an alternative to the moving done here. I can certainly do so, in case some non-negative reply comes back from him. > instead of hiding it again in what might look like an > unrelated function (there's no mention of PCI in acpi_iommu_init() > function name for example). Nor is there in acpi_mmcfg_init(). Irrespective of their names, both are firmly tied to PCI. Jan