On Wed, Aug 20, 2025 at 10:40 PM Troy Mitchell <troy.mitchell@xxxxxxxxx> wrote:
>
> On Wed, Aug 20, 2025 at 08:59:52PM +0800, fangyu.yu@xxxxxxxxxxxxxxxxx wrote:
> > From: "Guo Ren (Alibaba DAMO Academy)" <guoren@xxxxxxxxxx>
> >
> > The HGATP has been set to zero in gstage_mode_detect(), so there
> > is no need to save the old context. Unify the code convention
> > with gstage_mode_detect().
> >
> > Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@xxxxxxxxxx>
> > Signed-off-by: Fangyu Yu <fangyu.yu@xxxxxxxxxxxxxxxxx>
> > ---
> > arch/riscv/kvm/vmid.c | 5 +----
> > 1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/kvm/vmid.c b/arch/riscv/kvm/vmid.c
> > index 5f33625f4070..abb1c2bf2542 100644
> > --- a/arch/riscv/kvm/vmid.c
> > +++ b/arch/riscv/kvm/vmid.c
> > @@ -25,15 +25,12 @@ static DEFINE_SPINLOCK(vmid_lock);
> >
> > 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, (kvm_riscv_gstage_mode << HGATP_MODE_SHIFT) | 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);
> > + csr_write(CSR_HGATP, 0);
> Is setting HGATP to 0 in gstage_mode_detect meaningless now?
> If so, it might be better to drop it and just keep the one here.
Sorry, I misunderstood here.
1. kvm_riscv_gstage_vmid_detect() & gstage_mode_detect() are indepent
function, so keep csr_write(CSR_HGATP, 0) is considerable.
2. But your idea is good, because csr_write(CSR_HGATP, 0) would cause
TLB flush in some micro-arch, which reduces the IPC. So, removing
unnecessary CSR_write (CSR_HGATP, 0) is also considerable.
I would update V4 to merge kvm_riscv_gstage_vmid_detect() &
gstage_mode_detect() into a single function, inspired by your idea.
>
> - Troy
>
> >
> > /* We polluted local TLB so flush all guest TLB */
> > kvm_riscv_local_hfence_gvma_all();
> > --
> > 2.49.0
> >
> >
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--
Best Regards
Guo Ren
