linux-kernel - Re: [PATCH v12 08/20] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA

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Message-ID: <ZcMBDP6H5PRo5C2d@google.com>
Date: Tue, 6 Feb 2024 20:03:24 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paul Durrant <paul@....org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, David Woodhouse <dwmw2@...radead.org>, Shuah Khan <shuah@...nel.org>, 
	kvm@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v12 08/20] KVM: pfncache: allow a cache to be activated
 with a fixed (userspace) HVA

+s390 folks (question on kvm_is_error_gpa() for ya)

On Mon, Jan 15, 2024, Paul Durrant wrote:
> @@ -1398,7 +1414,9 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc);
>  static inline void kvm_gpc_mark_dirty(struct gfn_to_pfn_cache *gpc)
>  {
>  	lockdep_assert_held(&gpc->lock);
> -	mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
> +
> +	if (gpc->gpa != KVM_XEN_INVALID_GPA)

KVM_XEN_INVALID_GPA absolutely doesn't belong in common code.  Not to mention
that it will break when Paolo (rightly) moves it to an x86 header.

https://lore.kernel.org/all/20240131233056.10845-3-pbonzini@redhat.com

> +		mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
>  }
>  
>  void kvm_sigset_activate(struct kvm_vcpu *vcpu);
> diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
> index 97eec8ee3449..ae822bff812f 100644
> --- a/virt/kvm/pfncache.c
> +++ b/virt/kvm/pfncache.c
> @@ -48,7 +48,10 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
>  	if (!gpc->active)
>  		return false;
>  
> -	if (gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva))
> +	if (gpc->gpa != KVM_XEN_INVALID_GPA && gpc->generation != slots->generation)

This needs a comment.  I know what it's doing, but it wasn't obvious at first
glance, and it definitely won't be intuitive for readers that aren't intimately
familiar with memslots.

> +		return false;
> +
> +	if (kvm_is_error_hva(gpc->uhva))
>  		return false;
>  
>  	if (offset_in_page(gpc->uhva) + len > PAGE_SIZE)
> @@ -209,11 +212,13 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
>  	return -EFAULT;
>  }
>  
> -static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> +static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
>  			     unsigned long len)
>  {
>  	struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
> -	unsigned long page_offset = offset_in_page(gpa);
> +	unsigned long page_offset = (gpa != KVM_XEN_INVALID_GPA) ?
> +		offset_in_page(gpa) :
> +		offset_in_page(uhva);

This formatting is funky.  I also think it would be worth adding a helper to pair
with kvm_is_error_hva().

But!  kvm_is_error_gpa() already exists, and it very, very sneakily does a memslot
lookup and checks for a valid HVA.

s390 people, any objection to renaming kvm_is_error_gpa() to something like
kvm_gpa_has_memslot() or kvm_gpa_is_in_memslot()?  s390 is the only code that
uses the existing helper.

That would both to free up the name to pair with kvm_is_error_hva(), and would
make it obvious what the helper does; I was quite surprised that "error" means
"is covered by a valid memslot".

Back to this code, then we can have a slightly cleaner:

	unsigned long page_offset = kvm_is_error_gpa(gpa) ? offset_in_page(gpa) :
							    offset_in_page(uhva);


And I think it's worth asserting that exactly _one_ of GPA or HVA is valid, e.g.
to ensure KVM doesn't end up with botched offsets, and to make it a bit more
clear what's going on.


	if (WARN_ON_ONCE(kvm_is_error_gpa(gpa) == kvm_is_error_hva(uhva))
		return -EINVAL;