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

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Date: Wed, 14 Feb 2024 16:09:01 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: Sean Christopherson <seanjc@...gle.com>,
 David Woodhouse <dwmw2@...radead.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>,
 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

On 14/02/2024 16:01, Sean Christopherson wrote:
> On Tue, Feb 06, 2024, David Woodhouse wrote:
>> On Tue, 2024-02-06 at 20:03 -0800, Sean Christopherson wrote:
>>> +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
>>
>> We can use plain INVALID_GPA for that, I think. ISTR the reason we have
>> a separate KVM_XEN_INVALID_GPA is because that's a userspace API.
>>
>> ...
>>
>>> But!  kvm_is_error_gpa() already exists, and it very, very sneakily
>>> does a memslot lookup and checks for a valid HVA.
>>
>> Hm, that doesn't sound as fast as simple comparison. We also can't do
>> it from kvm_gpc_check(), can we?
> 
> You snipped the part where I suggested renaming the existing kvm_is_error_gpa().
> 
> I am suggesting we do the below (and obviously rename the s390 usage, too), and
> then the gpc code can use use kvm_is_error_gpa().
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index bbfefd7e612f..e1df988e4d57 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -148,6 +148,11 @@ static inline bool kvm_is_error_hva(unsigned long addr)
>   
>   #endif
>   
> +static inline bool kvm_is_error_gpa(gpa_t gpa)
> +{
> +       return gpa == INVALID_GPA;
> +}
> +

Are you ok with a local kvm_gpc_is_error_gpa() or somesuch until there 
is agreement with s390?

   Paul

>   #define KVM_ERR_PTR_BAD_PAGE   (ERR_PTR(-ENOENT))
>   
>   static inline bool is_error_page(struct page *page)
> @@ -1787,7 +1792,7 @@ static inline hpa_t pfn_to_hpa(kvm_pfn_t pfn)
>          return (hpa_t)pfn << PAGE_SHIFT;
>   }
>   
> -static inline bool kvm_is_error_gpa(struct kvm *kvm, gpa_t gpa)
> +static inline bool kvm_gpa_is_in_memslot(struct kvm *kvm, gpa_t gpa)
>   {
>          unsigned long hva = gfn_to_hva(kvm, gpa_to_gfn(gpa));
>