Re: [PATCH v3 3/3] x86/dom0: be less restrictive with the Interrupt Address Range

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Feb 20, 2025 at 09:33:46AM +0100, Jan Beulich wrote:
> On 19.02.2025 17:48, Roger Pau Monne wrote:
> > Xen currently prevents dom0 from creating CPU or IOMMU page-table mappings
> > into the interrupt address range [0xfee00000, 0xfeefffff].  This range has
> > two different purposes.  For accesses from the CPU is contains the default
> > position of local APIC page at 0xfee00000.  For accesses from devices
> > it's the MSI address range, so the address field in the MSI entries
> > (usually) point to an address on that range to trigger an interrupt.
> > 
> > There are reports of Lenovo Thinkpad devices placing what seems to be the
> > UCSI shared mailbox at address 0xfeec2000 in the interrupt address range.
> > Attempting to use that device with a Linux PV dom0 leads to an error when
> > Linux kernel maps 0xfeec2000:
> > 
> > RIP: e030:xen_mc_flush+0x1e8/0x2b0
> >  xen_leave_lazy_mmu+0x15/0x60
> >  vmap_range_noflush+0x408/0x6f0
> >  __ioremap_caller+0x20d/0x350
> >  acpi_os_map_iomem+0x1a3/0x1c0
> >  acpi_ex_system_memory_space_handler+0x229/0x3f0
> >  acpi_ev_address_space_dispatch+0x17e/0x4c0
> >  acpi_ex_access_region+0x28a/0x510
> >  acpi_ex_field_datum_io+0x95/0x5c0
> >  acpi_ex_extract_from_field+0x36b/0x4e0
> >  acpi_ex_read_data_from_field+0xcb/0x430
> >  acpi_ex_resolve_node_to_value+0x2e0/0x530
> >  acpi_ex_resolve_to_value+0x1e7/0x550
> >  acpi_ds_evaluate_name_path+0x107/0x170
> >  acpi_ds_exec_end_op+0x392/0x860
> >  acpi_ps_parse_loop+0x268/0xa30
> >  acpi_ps_parse_aml+0x221/0x5e0
> >  acpi_ps_execute_method+0x171/0x3e0
> >  acpi_ns_evaluate+0x174/0x5d0
> >  acpi_evaluate_object+0x167/0x440
> >  acpi_evaluate_dsm+0xb6/0x130
> >  ucsi_acpi_dsm+0x53/0x80
> >  ucsi_acpi_read+0x2e/0x60
> >  ucsi_register+0x24/0xa0
> >  ucsi_acpi_probe+0x162/0x1e3
> >  platform_probe+0x48/0x90
> >  really_probe+0xde/0x340
> >  __driver_probe_device+0x78/0x110
> >  driver_probe_device+0x1f/0x90
> >  __driver_attach+0xd2/0x1c0
> >  bus_for_each_dev+0x77/0xc0
> >  bus_add_driver+0x112/0x1f0
> >  driver_register+0x72/0xd0
> >  do_one_initcall+0x48/0x300
> >  do_init_module+0x60/0x220
> >  __do_sys_init_module+0x17f/0x1b0
> >  do_syscall_64+0x82/0x170
> > 
> > Remove the restrictions to create mappings the interrupt address range for
> 
> Nit: Missing "in"?

Indeed, thanks for spotting.

> > dom0.  Note that the restriction to map the local APIC page is enforced
> > separately, and that continues to be present.  Additionally make sure the
> > emulated local APIC page is also not mapped, in case dom0 is using it.
> 
> But that's in GFN space, not in MFN one. Why would that matter for iomem_caps?

It's required to avoid arch_iommu_hwdom_init() creating an identity
mapping for APIC_DEFAULT_PHYS_BASE, which would prevent the local APIC
emulation from being used.

Note that mp_lapic_addr can be zeor if the host local APICs are
started in x2APIC mode, or it could (in theory) contain an address
different than APIC_DEFAULT_PHYS_BASE.

Thanks, Roger.