[SRU][K][PATCH 0/1] Disable sv57 on riscv64

[Emil Renner Berthing]

[ Impact ]

* Linux 5.18 merged support for 5-level page tables (sv57), and it
  does it such that 5-level page tables are used whenever the
  system supports it.

* So far only Qemu support sv57, but it breaks Go since Go uses
  too many of the upper bits of pointers to store data, and is not
  yet ready for sv57. It probably breaks other types of software
  using too many bits for this "pointer tagging" technique.
  At the moment, there is no way to indicate to the kernel to
  downgrade to sv48, so we must patch the kernel:

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 2c4a64e97aec..18a0c70ed313 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -775,6 +775,10 @@ static __init void set_satp_mode(void)
                disable_pgtable_l4();
        }

+ /* UBUNTU: Force disable sv57 and fallback to sv48 */
+ if (pgtable_l5_enabled)
+ disable_pgtable_l5();
+
        memset(early_pg_dir, 0, PAGE_SIZE);
        memset(early_p4d, 0, PAGE_SIZE);
        memset(early_pud, 0, PAGE_SIZE);

[ Test Plan ]

* Build an image with the patched kernel and make sure snapd and
  other Go programs works correctly and that you can compile a
  simple helloworld written in Go.

[ Where problems could occur ]

* The patch could be buggy.

* Qemu will no longer work with > 256 TB of memory.

Alexandre Ghiti (1):
  UBUNTU: SAUCE: riscv: mm: Force disable sv57

 arch/riscv/mm/init.c | 4 ++++
 1 file changed, 4 insertions(+)

-- 
2.37.2