On Mon, Dec 12, 2022 at 8:31 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote: > > During the early page table creation, we used to set the mapping for > PAGE_OFFSET to the kernel load address: but the kernel load address is > always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD > pages as this physical address is not aligned on PUD/P4D/PGD size (whereas > PAGE_OFFSET is). > > But actually we don't have to establish this mapping (ie set va_pa_offset) > that early in the boot process because: > > - first, setup_vm installs a temporary kernel mapping and among other > things, discovers the system memory, > - then, setup_vm_final creates the final kernel mapping and takes > advantage of the discovered system memory to create the linear > mapping. > > During the first phase, we don't know the start of the system memory and > then until the second phase is finished, we can't use the linear mapping at > all and phys_to_virt/virt_to_phys translations must not be used because it > would result in a different translation from the 'real' one once the final > mapping is installed. > > So here we simply delay the initialization of va_pa_offset to after the > system memory discovery. But to make sure noone uses the linear mapping > before, we add some guard in the DEBUG_VIRTUAL config. > > Finally we can use PUD/P4D/PGD hugepages when possible, which will result > in a better TLB utilization. > > Note that we rely on the firmware to protect itself using PMP. > > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> > --- > > v2: > - Add a comment on why RISCV64 does not need to set initrd_start/end that > early in the boot process, as asked by Rob > > Note that this patch is rebased on top of: > [PATCH v1 1/1] riscv: mm: call best_map_size many times during linear-mapping > > arch/riscv/include/asm/page.h | 16 ++++++++++++++++ > arch/riscv/mm/init.c | 25 +++++++++++++++++++------ > arch/riscv/mm/physaddr.c | 16 ++++++++++++++++ > drivers/of/fdt.c | 7 ++++++- > 4 files changed, 57 insertions(+), 7 deletions(-) [...] > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c > index 7b571a631639..012554445054 100644 > --- a/drivers/of/fdt.c > +++ b/drivers/of/fdt.c > @@ -895,8 +895,13 @@ static void __early_init_dt_declare_initrd(unsigned long start, > * enabled since __va() is called too early. ARM64 does make use > * of phys_initrd_start/phys_initrd_size so we can skip this > * conversion. > + * On RISCV64, the usage of __va() before the linear mapping exists > + * is wrong and RISCV64 rightly calls reserve_initrd_mem after it is > + * available where it actually resets the translation that is done > + * here and re-computes it. No, I want a single comment that covers both cases (and the next arch we add here). Something like this: __va() is not yet available this early on some platforms. In that case, the platform uses phys_initrd_start/phys_initrd_size instead and does the VA conversion itself. Rob
