[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] hvmloader: pass PCI MMIO layout to OVMF as an info table
On 11.01.2021 15:49, Laszlo Ersek wrote: > On 01/11/21 15:00, Igor Druzhinin wrote: >> On 11/01/2021 09:27, Jan Beulich wrote: >>> On 11.01.2021 05:53, Igor Druzhinin wrote: >>>> We faced a problem with passing through a PCI device with 64GB BAR to >>>> UEFI guest. The BAR is expectedly programmed into 64-bit PCI aperture at >>>> 64G address which pushes physical address space to 37 bits. OVMF uses >>>> address width early in PEI phase to make DXE identity pages covering >>>> the whole addressable space so it needs to know the last address it needs >>>> to cover but at the same time not overdo the mappings. >>>> >>>> As there is seemingly no other way to pass or get this information in >>>> OVMF at this early phase (ACPI is not yet available, PCI is not yet >>>> enumerated, >>>> xenstore is not yet initialized) - extend the info structure with a new >>>> table. Since the structure was initially created to be extendable - >>>> the change is backward compatible. >>> >>> How does UEFI handle the same situation on baremetal? I'd guess it is >>> in even more trouble there, as it couldn't even read addresses from >>> BARs, but would first need to assign them (or at least calculate >>> their intended positions). >> >> Maybe Laszlo or Anthony could answer this question quickly while I'm >> investigating? > > On the bare metal, the phys address width of the processor is known. >From CPUID I suppose. > OVMF does the whole calculation in reverse because there's no way for it > to know the physical address width of the physical (= host) CPU. > "Overdoing" the mappings doesn't only waste resources, it breaks hard > with EPT -- access to a GPA that is inexpressible with the phys address > width of the host CPU (= not mappable successfully with the nested page > tables) will behave super bad. I don't recall the exact symptoms, but it > prevents booting the guest OS. > > This is why the most conservative 36-bit width is assumed by default. IOW you don't trust virtualized CPUID output? Jan
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