linux-kernel - Re: [PATCH V2] RISC-V: KVM: Write hgatp register with valid mode bits

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Message-ID: <CAJF2gTTqPMVTNdHL7PUwobXQr3dwzKPi13ZDpmkVz+3VXHLZVw@mail.gmail.com>
Date: Mon, 18 Aug 2025 14:17:55 +0800
From: Guo Ren <guoren@...nel.org>
To: fangyu.yu@...ux.alibaba.com
Cc: anup@...infault.org, atish.patra@...ux.dev, paul.walmsley@...ive.com, 
	palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr, 
	guoren@...ux.alibaba.com, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] RISC-V: KVM: Write hgatp register with valid mode bits

On Mon, Aug 18, 2025 at 1:42 PM <fangyu.yu@...ux.alibaba.com> wrote:
>
> From: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
>
> According to the RISC-V Privileged Architecture Spec, when MODE=Bare
> is selected,software must write zero to the remaining fields of hgatp.
>
> We have detected the valid mode supported by the HW before, So using a
> valid mode to detect how many vmid bits are supported.
Good catch! It's a bug. The code seems copied from asids_init(), whose
old value is not bare mode. For real hardware, it would cause
problems, but the qemu buggy code hides the problem.

It needs a tag: Fixes: fd7bb4a251df ("RISC-V: KVM: Implement VMID allocator")

Others, Reviewed-by: Guo Ren <guoren@...enl.org>

>
> Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
>
> ---
> Changes in v2:
> - Fixed build error since kvm_riscv_gstage_mode() has been modified.
> ---
>  arch/riscv/kvm/vmid.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kvm/vmid.c b/arch/riscv/kvm/vmid.c
> index 3b426c800480..5f33625f4070 100644
> --- a/arch/riscv/kvm/vmid.c
> +++ b/arch/riscv/kvm/vmid.c
> @@ -14,6 +14,7 @@
>  #include <linux/smp.h>
>  #include <linux/kvm_host.h>
>  #include <asm/csr.h>
> +#include <asm/kvm_mmu.h>
>  #include <asm/kvm_tlb.h>
>  #include <asm/kvm_vmid.h>
>
> @@ -28,7 +29,7 @@ void __init kvm_riscv_gstage_vmid_detect(void)
>
>         /* Figure-out number of VMID bits in HW */
>         old = csr_read(CSR_HGATP);
> -       csr_write(CSR_HGATP, old | HGATP_VMID);
> +       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);
> --
> 2.49.0
>


-- 
Best Regards
 Guo Ren