linux-kernel - Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in writable memselot

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Message-ID: <CAHBxVyHFkNtFdX-vciPvYnTOH=GXvHVW7hjFrLA4MFr9wqWVvQ@mail.gmail.com>
Date: Wed, 3 Sep 2025 23:24:24 -0700
From: Atish Kumar Patra <atishp@...osinc.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Anup Patel <anup@...infault.org>, Will Deacon <will@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Mayuresh Chitale <mchitale@...tanamicro.com>, 
	linux-riscv@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	kvm-riscv@...ts.infradead.org
Subject: Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in
 writable memselot

On Fri, Aug 29, 2025 at 1:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Fri, Aug 29, 2025, Atish Patra wrote:
> > The arch specific code may need to know if a particular gpa is valid and
> > writable for the shared memory between the host and the guest. Currently,
> > there are few places where it is used in RISC-V implementation. Given the
> > nature of the function it may be used for other architectures.
> > Hence, a common helper function is added.
> >
> > Suggested-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Atish Patra <atishp@...osinc.com>
> > ---
> >  include/linux/kvm_host.h | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 15656b7fba6c..eec5cbbcb4b3 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1892,6 +1892,14 @@ static inline bool kvm_is_gpa_in_memslot(struct kvm *kvm, gpa_t gpa)
> >       return !kvm_is_error_hva(hva);
> >  }
> >
> > +static inline bool kvm_is_gpa_in_writable_memslot(struct kvm *kvm, gpa_t gpa)
> > +{
> > +     bool writable;
> > +     unsigned long hva = gfn_to_hva_prot(kvm, gpa_to_gfn(gpa), &writable);
> > +
> > +     return !kvm_is_error_hva(hva) && writable;
>
> I don't hate this API, but I don't love it either.  Because knowing that the
> _memslot_ is writable doesn't mean all that much.  E.g. in this usage:
>
>         hva = kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &writable);
>         if (kvm_is_error_hva(hva) || !writable)
>                 return SBI_ERR_INVALID_ADDRESS;
>
>         ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta));
>         if (ret)
>                 return SBI_ERR_FAILURE;
>
> the error code returned to the guest will be different if the memslot is read-only
> versus if the VMA is read-only (or not even mapped!).  Unless every read-only
> memslot is explicitly communicated as such to the guest, I don't see how the guest
> can *know* that a memslot is read-only, so returning INVALID_ADDRESS in that case
> but not when the underlying VMA isn't writable seems odd.
>
> It's also entirely possible the memslot could be replaced with a read-only memslot
> after the check, or vice versa, i.e. become writable after being rejected.  Is it
> *really* a problem to return FAILURE if the guest attempts to setup steal-time in
> a read-only memslot?  I.e. why not do this and call it good?
>

Reposting the response as gmail converted my previous response as
html. Sorry for the spam.

>From a functionality pov, that should be fine. However, we have
explicit error conditions for read only memory defined in the SBI STA
specification[1].
Technically, we will violate the spec if we return FAILURE instead of
INVALID_ADDRESS for read only memslot.

TBH, I don't save much duplicate code with the new generic API now.
If you don't see if the generic API will be useful in other cases, I
can drop that patch and changes in the steal time code.

[1] https://github.com/riscv-non-isa/riscv-sbi-doc/blob/master/src/ext-steal-time.adoc#table_sta_steal_time_set_shmem_errors
>         if (!kvm_is_gpa_in_memslot(vcpu->kvm, shmem >> PAGE_SHIFT))
>                 return SBI_ERR_INVALID_ADDRESS;
>
>         ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta));
>         if (ret)
>                 return SBI_ERR_FAILURE;