On 12/22/22 17:21, Conor Dooley wrote: > 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 That's because my CONFIG_DEBUG_VIRTUAL fix got applied first. This patch applies cleanly after: git revert -m1 61b2f0bdaa3c7e6956fdac0a7c1e8284b9b81d1d though you'll get one trivial conflict if you try to undo the revert. Regards, Samuel
