On 24.06.2025 12:33, Oleksii Kurochko wrote:
> On 6/18/25 5:15 PM, Jan Beulich wrote:
>> On 10.06.2025 15:05, Oleksii Kurochko wrote:
>>> Instruct the remote harts to execute one or more HFENCE.GVMA instructions,
>>> covering the range of guest physical addresses between start_addr and
>>> start_addr + size for all the guests.
>> Here and in the code comment: Why "for all the guests"? Under what conditions
>> would you require such a broad (guest) TLB flush?
>
> Hmm, it seems like KVM always do such a broad (guest) TLB flush during
> detection
> of VMIDLEN:
> void __init kvm_riscv_gstage_vmid_detect(void)
> {
> unsigned long old;
>
> /* Figure-out number of VMID bits in HW */
> old = csr_read(CSR_HGATP);
> csr_write(CSR_HGATP, old | HGATP_VMID);
> vmid_bits = csr_read(CSR_HGATP);
> vmid_bits = (vmid_bits & HGATP_VMID) >> HGATP_VMID_SHIFT;
> vmid_bits = fls_long(vmid_bits);
> csr_write(CSR_HGATP, old);
>
> /* We polluted local TLB so flush all guest TLB */
> kvm_riscv_local_hfence_gvma_all();
>
> /* We don't use VMID bits if they are not sufficient */
> if ((1UL << vmid_bits) < num_possible_cpus())
> vmid_bits = 0;
> }
>
> It is not clear actually why so broad and why not hfence_gvma_vmid(vmid_bits).
>
> And I am not really 100% sure that any hfence_gvma() is needed here as I
> don't see
> what could pollutes local guest TLB between csr_write() calls.
>
> RISC-V spec. says that:
> Note that writing hgatp does not imply any ordering constraints between
> page-table updates and
> subsequent G-stage address translations. If the new virtual machine’s
> guest physical page tables have
> been modified, or if a VMID is reused, it may be necessary to execute
> an HFENCE.GVMA instruction
> (see Section 18.3.2) before or after writing hgatp.
>
> But we don't modify VM's guest physical page table. We could potentially
> reuse VMID between csr_write()
> calls, but it is returning back and we don't switch to a guest with this
> "new" VMID, so it isn't really used.
That would be my expectation, too. Yet I don't know if RISC-V has any
peculiarities there.
Jan
