linux-kernel - Re: [PATCH v7 05/11] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA

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Date:   Thu, 2 Nov 2023 18:01:35 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paul Durrant <pdurrant@...zon.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v7 05/11] KVM: pfncache: allow a cache to be activated
 with a fixed (userspace) HVA

On 31/10/2023 23:49, Sean Christopherson wrote:
> On Mon, Oct 02, 2023, Paul Durrant wrote:
>> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
>> index 6f4737d5046a..d49946ee7ae3 100644
>> --- a/include/linux/kvm_types.h
>> +++ b/include/linux/kvm_types.h
>> @@ -64,7 +64,7 @@ struct gfn_to_hva_cache {
>>   
>>   struct gfn_to_pfn_cache {
>>   	u64 generation;
>> -	gpa_t gpa;
>> +	u64 addr;
> 
> Holy moly, we have unions for exactly this reason.
> 
> 	union {
> 		gpa_t gpa;
> 		unsigned long addr;
> 	};
> 
> But that's also weird and silly because it's basically the exact same thing as
> "uhva".  If "uhva" stores the full address instead of the page-aligned address,
> then I don't see a need for unionizing the gpa and uhva.
> 

Ok, I think that'll be more invasive but I'll see how it looks.

> kvm_xen_vcpu_get_attr() should darn well explicitly check that the gpc stores
> the correct type and not bleed ABI into the gfn_to_pfn_cache implementation.
> 

I guess if we leave gpa alone and make it INVALID_GPA for caches 
initialized using an HVA then that can be checked. Is that what you mean 
here?

> If there's a true need for a union, the helpers should WARN.
> 
>> +unsigned long kvm_gpc_hva(struct gfn_to_pfn_cache *gpc)
>> +{
>> +	return !gpc->addr_is_gpa ? gpc->addr : 0;
> 
> '0' is a perfectly valid address.  Yeah, practically speaking '0' can't be used
> these days, but we already have KVM_HVA_ERR_BAD.  If y'all want to use the for the
> Xen ABI, then so be it.  But the common helpers need to use a sane value.

Ok.

   Paul