On Tue, Dec 13, 2022 at 07:02:04AM +0100, Alexandre Ghiti 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> > --- > > v3: > - Change the comment about initrd_start VA conversion so that it fits > ARM64 and RISCV64 (and others in the future if needed), as suggested > by Rob > > 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 Hey Alex, unfortunately I could not get this to apply either (I tried a riscv/for-next & Linus' tree). The above patch should be in both, so idk: git am -3 v3_20221213_alexghiti_riscv_use_pud_p4d_pgd_pages_for_the_linear_mapping.mbx Applying: riscv: Use PUD/P4D/PGD pages for the linear mapping error: sha1 information is lacking or useless (arch/riscv/mm/init.c). error: could not build fake ancestor Patch failed at 0001 riscv: Use PUD/P4D/PGD pages for the linear mapping I assume it'll need a rebase after -rc1? It's really nice to see someone looking at this early code, from the bit of digging around that I've done chasing bugs it feels like it needs it- so thanks!
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