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: <ec89ab12288426761ab5bd7d05562a4e8834e5f1.camel@infradead.org>
Date:   Tue, 21 Nov 2023 22:47:47 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <paul@....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 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? 

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?

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