On Fri, 3 Nov 2023 at 15:21, Peter Maydell <peter.mayd...@linaro.org> wrote: > > From: Udo Steinberg <u...@hypervisor.org> > > Documentation for using the GAS in ACPI tables to report debug UART addresses > at > https://learn.microsoft.com/en-us/windows-hardware/drivers/bringup/acpi-debug-port-table > states the following: > > - The Register Bit Width field contains the register stride and must be a > power of 2 that is at least as large as the access size. On 32-bit > platforms this value cannot exceed 32. On 64-bit platforms this value > cannot exceed 64. > - The Access Size field is used to determine whether byte, WORD, DWORD, or > QWORD accesses are to be used. QWORD accesses are only valid on 64-bit > architectures. > > Documentation for the ARM PL011 at > https://developer.arm.com/documentation/ddi0183/latest/ > states that the registers are: > > - spaced 4 bytes apart (see Table 3-2), so register stride must be 32. > - 16 bits in size in some cases (see individual registers), so access > size must be at least 2. > > Linux doesn't seem to care about this error in the table, but it does > affect at least the NOVA microhypervisor. > > In theory we therefore have a choice between reporting the access > size as 2 (16 bit accesses) or 3 (32-bit accesses). In practice, > Linux does not correctly handle the case where the table reports the > access size as 2: as of kernel commit 750b95887e5678, the code in > acpi_parse_spcr() tries to tell the serial driver to use 16 bit > accesses by passing "mmio16" in the option string, but the PL011 > driver code in pl011_console_match() only recognizes "mmio" or > "mmio32". The result is that unless the user has enabled 'earlycon' > Oops, a line seems to have got deleted here -- should continue
"there is no console output from the guest kernel." > We therefore choose to report the access size as 32 bits; this works > for NOVA and also for Linux. It is also what the UEFI firmware on a > Raspberry Pi 4 reports, so we're in line with existing real-world > practice. > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1938 > Signed-off-by: Udo Steinberg <u...@hypervisor.org> > Reviewed-by: Peter Maydell <peter.mayd...@linaro.org> > [PMM: minor commit message tweaks; use 32 bit accesses] > Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> > -- thanks -- PMM
