linux-kernel - Re: [PATCH v8 08/15] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA

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Message-ID: <5e0e13ea-3eee-41bd-a070-e7bd9ed5d2e9@xen.org>
Date:   Wed, 22 Nov 2023 10:07:00 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        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>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 08/15] KVM: pfncache: allow a cache to be activated
 with a fixed (userspace) HVA

On 21/11/2023 22:47, David Woodhouse wrote:
> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
>>
>> -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, u64 addr, bool addr_is_gpa,
>>                               unsigned long len)
>>   {
>>          struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
>> -       unsigned long page_offset = offset_in_page(gpa);
>> +       unsigned long page_offset = offset_in_page(addr);
>>          bool unmap_old = false;
>>          kvm_pfn_t old_pfn;
>>          bool hva_change = false;
>> @@ -244,12 +244,21 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
>>          old_pfn = gpc->pfn;
>>          old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
>>   
>> -       /* If the userspace HVA is invalid, refresh that first */
>> -       if (gpc->gpa != gpa || gpc->generation != slots->generation ||
>> -           kvm_is_error_hva(gpc->uhva)) {
>> -               gfn_t gfn = gpa_to_gfn(gpa);
>> +       if (!addr_is_gpa) {
>> +               gpc->gpa = KVM_XEN_INVALID_GPA;
>> +               gpc->uhva = PAGE_ALIGN_DOWN(gpc->uhva);
>> +               addr = PAGE_ALIGN_DOWN(addr);
>> +
>> +               if (gpc->uhva != addr) {
>> +                       gpc->uhva = addr;
>> +                       hva_change = true;
>> +               }
>> +       } else if (gpc->gpa != addr ||
>> +                  gpc->generation != slots->generation ||
>> +                  kvm_is_error_hva(gpc->uhva)) {
>> +               gfn_t gfn = gpa_to_gfn(addr);
>>   
>> -               gpc->gpa = gpa;
>> +               gpc->gpa = addr;
>>                  gpc->generation = slots->generation;
>>                  gpc->memslot = __gfn_to_memslot(slots, gfn);
>>                  gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
> 
> Hrm, now that a previous patch means we're preserving the low bits of
> gpc->uhva surely you don't *need* to mess with the gpc struct?
> 

I'm not messing with it, am I?

> If gpc->gpa == KVM_XEN_INVALID_GPA (but gpc->uhva != KVM_ERR_ERR_BAD &&
> gpc->active) surely that's enough to signal that gpc->uhva is canonical
> and doesn't need to be looked up from the GPA?
> 
> And I think that means the 'bool addr_is_gpa' argument can go away from
> __kvm_gpc_refresh(); you can set it up in {__,}kvm_gpc_activate*()
> instead?

Alas not... __kvm_gpc_refresh() still needs to know *something* has 
changed, otherwise the khva will be stale.

   Paul