linux-kernel - Re: [PATCH v7 03/11] KVM: pfncache: add a helper to get the gpa

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Message-ID: <ZUGPosqRPNf155sX@google.com>
Date:   Tue, 31 Oct 2023 16:37:06 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paul Durrant <paul@....org>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paul Durrant <pdurrant@...zon.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v7 03/11] KVM: pfncache: add a helper to get the gpa

On Mon, Oct 02, 2023, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@...zon.com>
> 
> A subsequent patch will rename this field since it will become overloaded.

Too. Many. Pronouns.

  Add a helper to get the gpa of a gpc cache, as a subsequent patch will
  rename "gpa" to "addr".

> To avoid churn in places that currently retrieve the gpa, add a helper for
> that purpose now.

This is silly.  If this series added any protection against incorrect usage then
I could understand the helper, but this just end up being

	return gpc->addr_is_gpa ? gpc->addr : INVALID_GPA;

which is nasty.  IIUC, there's no WARN because kvm_xen_vcpu_get_attr() doesn't
pre-check that the cache is in the correct mode.  That's a really silly reason
to not harden the rest of KVM.

> diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
> index b68ed7fa56a2..17afbb464a70 100644
> --- a/virt/kvm/pfncache.c
> +++ b/virt/kvm/pfncache.c
> @@ -386,6 +386,12 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
>  }
>  EXPORT_SYMBOL_GPL(kvm_gpc_activate);
>  
> +gpa_t kvm_gpc_gpa(struct gfn_to_pfn_cache *gpc)
> +{
> +	return gpc->gpa;
> +}
> +EXPORT_SYMBOL_GPL(kvm_gpc_gpa);

Any reason not to make this static inline?  Even in the final form, not making
this inlined seems silly.

Belatedly, same question for kvm_gpc_mark_dirty() I suppose.

>  void kvm_gpc_mark_dirty(struct gfn_to_pfn_cache *gpc)
>  {
>  	mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
> -- 
> 2.39.2
>