risc-v pmp implementation error

Bug #1886076 reported by Nicolas Royer on 2020-07-02
6
This bug affects 1 person
Affects Status Importance Assigned to Milestone
QEMU
Undecided
Alistair Francis

Bug Description

QEMU Commit fc1bff958998910ec8d25db86cd2f53ff125f7ab

RISC-V PMP implementation is not correct on QEMU.

When an access is granted there is no more PMP check on the 4KB memory range of the accessed location.
A cache flush is needed in order to force a PMP check on next access to this 4KB memory range.
A correct implementation would be to grant access to the maximum allowed area around the accessed location within the 4KB memory range.

For instance, if PMP is configured to block all accesses from 0x80003000 to 0x800037FF and from 0x80003C00 to 0x80003FFF:
1st case:
    1) A read access is done @0x80003900 --> access OK as expected
    2) Then a read access is done @0x80003400 --> access OK while it must be blocked!
2nd case:
    1) A read access is done @0x80003900 --> access OK as expected
    2) Cache is flushed (__asm__ __volatile__ ("sfence.vma" : : : "memory");)
    3) A read access is done @0x80003400 --> access blocked as expected

Analysis:
    After the 1st read @0x80003900 QEMU add the memory range 0x80003000 to 0x80003FFF into a TLB entry.
    Then no more PMP check is done from 0x80003000 to 0x80003FFF until the TLB is flushed.
What should be done:
    Only the range 0x80003800 to 0x80003BFF should be added to the TLB entry.

The 4KB range is the default size of a TLB page on QEMU for RISCV.
The minimum size that can be set is 64Bytes. However the PMP granularity can be as low as 4Bytes.

I tested a quick fix and PMP is working as expected.
The quick fix consist in replacing this line:
tlb_set_page(cs, address & TARGET_PAGE_MASK, pa & TARGET_PAGE_MASK, prot, mmu_idx, TARGET_PAGE_SIZE);
By this one in target/riscv/cpu_helper.c:
tlb_set_page(cs, address & ~0x3, pa & ~0x3, prot, mmu_idx, size);

This quick fix has to be optimized in order to consume less HW resources, as explained at the beginning.

Laurent Vivier (laurent-vivier) wrote :

Nicolas,

to be reviewed your patch must be sent to <email address hidden>

Alistair Francis (alistair2323) wrote :

This should be fixed once the current RISC-V branch is merged into master.

You can see the patch that fixes this here: https://<email address hidden><email address hidden>/

Changed in qemu:
assignee: nobody → Alistair Francis (alistair2323)
Alistair Francis (alistair2323) wrote :

I'm marking this as fix committed, although the fix isn't yet in master it's in the RISC-V tree and will be in master soon.

Changed in qemu:
status: New → Fix Committed
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