[PATCH] riscv: kvm: Skip preventive hfence.gvma for new G-stage mappings with Svvptc

[PATCH] riscv: kvm: Skip preventive hfence.gvma for new G-stage mappings with Svvptc

[Dylan.Wu]

For Invalid->Valid G-stage leaf PTE updates, the flush in
kvm_riscv_gstage_set_pte() is preventive. When Svvptc is available, new
valid PTEs become visible to the page-table walker within a bounded
time, so an immediate hfence.gvma is unnecessary.

Skip gstage_tlb_flush() for that case. Permission-restricting updates
continue to use the existing flush path.

Assisted-by: YuanSheng: deepseek-v4-pro
Co-developed-by: Quan Zhou <zhouquan@iscas.ac.cn>
Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
Signed-off-by: Dylan.Wu <fredwudi0305@gmail.com>
---
 arch/riscv/kvm/gstage.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kvm/gstage.c b/arch/riscv/kvm/gstage.c
index c4c3b7956..7b805112d 100644
--- a/arch/riscv/kvm/gstage.c
+++ b/arch/riscv/kvm/gstage.c
@@ -10,6 +10,8 @@
 #include <linux/module.h>
 #include <linux/pgtable.h>
 #include <asm/kvm_gstage.h>
+#include <asm/cpufeature.h>
+#include <asm/hwcap.h>
 
 #ifdef CONFIG_64BIT
 unsigned long kvm_riscv_gstage_max_pgd_levels __ro_after_init = 3;
@@ -171,8 +173,11 @@ int kvm_riscv_gstage_set_pte(struct kvm_gstage *gstage,
 	}
 
 	if (pte_val(*ptep) != pte_val(map->pte)) {
+		bool was_invalid = !pte_val(*ptep);
 		set_pte(ptep, map->pte);
-		if (gstage_pte_leaf(ptep))
+		if (gstage_pte_leaf(ptep) &&
+		    !(was_invalid &&
+		      riscv_has_extension_unlikely(RISCV_ISA_EXT_SVVPTC)))
 			gstage_tlb_flush(gstage, current_level, map->addr);
 	}
 
-- 
2.34.1

Re: [PATCH] riscv: kvm: Skip preventive hfence.gvma for new G-stage mappings with Svvptc

[Anup Patel]

On Wed, Jul 1, 2026 at 1:23 PM Dylan.Wu <fredwudi0305@gmail.com> wrote:
>
> For Invalid->Valid G-stage leaf PTE updates, the flush in
> kvm_riscv_gstage_set_pte() is preventive. When Svvptc is available, new
> valid PTEs become visible to the page-table walker within a bounded
> time, so an immediate hfence.gvma is unnecessary.
>
> Skip gstage_tlb_flush() for that case. Permission-restricting updates
> continue to use the existing flush path.
>
> Assisted-by: YuanSheng: deepseek-v4-pro
> Co-developed-by: Quan Zhou <zhouquan@iscas.ac.cn>
> Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
> Signed-off-by: Dylan.Wu <fredwudi0305@gmail.com>

The patch subject is unnecessarily long and needs some simplification.
Also, there is minor warning reported by checkpatch.pl related to
include of asm/cpufeature.h. I will take care of both of these at the
time of merging this patch.

Reviewed-by: Anup Patel <anup@brainfault.org>

Queued this patch as a fix for Linux-7.2-rcX

Thanks,
Anup

> ---
>  arch/riscv/kvm/gstage.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kvm/gstage.c b/arch/riscv/kvm/gstage.c
> index c4c3b7956..7b805112d 100644
> --- a/arch/riscv/kvm/gstage.c
> +++ b/arch/riscv/kvm/gstage.c
> @@ -10,6 +10,8 @@
>  #include <linux/module.h>
>  #include <linux/pgtable.h>
>  #include <asm/kvm_gstage.h>
> +#include <asm/cpufeature.h>
> +#include <asm/hwcap.h>
>
>  #ifdef CONFIG_64BIT
>  unsigned long kvm_riscv_gstage_max_pgd_levels __ro_after_init = 3;
> @@ -171,8 +173,11 @@ int kvm_riscv_gstage_set_pte(struct kvm_gstage *gstage,
>         }
>
>         if (pte_val(*ptep) != pte_val(map->pte)) {
> +               bool was_invalid = !pte_val(*ptep);
>                 set_pte(ptep, map->pte);
> -               if (gstage_pte_leaf(ptep))
> +               if (gstage_pte_leaf(ptep) &&
> +                   !(was_invalid &&
> +                     riscv_has_extension_unlikely(RISCV_ISA_EXT_SVVPTC)))
>                         gstage_tlb_flush(gstage, current_level, map->addr);
>         }
>
> --
> 2.34.1
>