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

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Message-ID: <ZUGScpSFlojjloQk@google.com>
Date:   Tue, 31 Oct 2023 16:49:06 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paul Durrant <paul@....org>
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 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.

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.

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.