linux-kernel - Re: [PATCH v13 12/21] KVM: x86/xen: allow shared

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Date: Tue, 20 Feb 2024 09:03:30 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>,
 David Hildenbrand <david@...hat.com>, Heiko Carstens <hca@...ux.ibm.com>,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>,
 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-s390@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v13 12/21] KVM: x86/xen: allow shared_info to be mapped by
 fixed HVA

On 19/02/2024 21:53, Sean Christopherson wrote:
> On Thu, Feb 15, 2024, Paul Durrant wrote:
>> @@ -715,13 +731,23 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
>>   		break;
>>   
>>   	case KVM_XEN_ATTR_TYPE_SHARED_INFO:
>> -		if (kvm->arch.xen.shinfo_cache.active)
>> +		if (kvm->arch.xen.shinfo_cache.active &&
>> +		    kvm->arch.xen.shinfo_cache.gpa != KVM_XEN_INVALID_GPA)
> 
> This should really use INVALID_GPA when checking internal gpc state.  Mostly to
> help clarify what is/isn't KVM Xen ABI, but also because I don't like the subtle
> assumption that KVM_XEN_INVALID_GPA == INVALID_GPA.
> 

Sorry, yes, that was miss on my part.

> Even better, if we slot in two helpers when the HVA-based GPC support is added,
> then the Xen code doesn't need to to make assumptions about how the GPC code
> manages HVA vs. GPA internally.  E.g. if we ever refactor the code again and use
> a dedicated flag instead of gpc->gpa as the implicit flag.
> 
>    static inline bool kvm_gpc_is_gpa_active(struct gfn_to_pfn_cache *gpc)
>    {
> 	return gpc->active && !kvm_is_error_gpa(gpc->gpa);
>    }
> 
>    static inline bool kvm_gpc_is_hva_active(struct gfn_to_pfn_cache *gpc)
>    {
> 	return gpc->active && kvm_is_error_gpa(gpc->gpa);
>    }

Sure, sounds good to me. I'll add those.