[PATCH v3 2/2] RISC-V KVM: Remove unnecessary HGATP csr_read

[Guo Ren]

On Thu, Aug 21, 2025 at 9:25 AM Guo Ren <guoren at kernel.org> wrote:
>
> On Wed, Aug 20, 2025 at 10:40 PM Troy Mitchell <troy.mitchell at linux.dev> wrote:
> >
> > On Wed, Aug 20, 2025 at 08:59:52PM +0800, fangyu.yu at linux.alibaba.com wrote:
> > > From: "Guo Ren (Alibaba DAMO Academy)" <guoren at kernel.org>
> > >
> > > 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 at kernel.org>
> > > Signed-off-by: Fangyu Yu <fangyu.yu at linux.alibaba.com>
> > > ---
> > >  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.
I found we can't remove "csr_write(CSR_HGATP, 0)" in
gstage_mode_detect(), because this patch needs to reset the HGATP when
mode check failure:
https://lore.kernel.org/linux-riscv/20250819004643.1884149-1-guoren@kernel.org/

I would update to v4, which collects these related patches together
for review convenience.

>
> >
> >                 - Troy
> >
> > >
> > >       /* We polluted local TLB so flush all guest TLB */
> > >       kvm_riscv_local_hfence_gvma_all();
> > > --
> > > 2.49.0
> > >
> > >
> > > _______________________________________________
> > > linux-riscv mailing list
> > > linux-riscv at lists.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/linux-riscv
>
>
>
> --
> Best Regards
>  Guo Ren



-- 
Best Regards
 Guo Ren