linux-kernel - Re: [PATCH] x86/virt/tdx: Use precalculated TDVPR page physical address

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Message-ID: <aPZUY90M0B3Tu3no@google.com>
Date: Mon, 20 Oct 2025 08:25:23 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, "Kirill A. Shutemov" <kas@...nel.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Kai Huang <kai.huang@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Thomas Huth <thuth@...hat.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, linux-coco@...ts.linux.dev, kvm@...r.kernel.org, 
	Farrah Chen <farrah.chen@...el.com>
Subject: Re: [PATCH] x86/virt/tdx: Use precalculated TDVPR page physical address

On Mon, Oct 20, 2025, Dave Hansen wrote:
> On 10/20/25 07:42, Sean Christopherson wrote:
> >> In a perfect world, we'd have sparse annotations for the vaddr, paddr,
> >> pfn, dma_addr_t and all the other address spaces. Until then, I like
> >> passing struct page around.
> > But that clearly doesn't work since now the raw paddr is being passed in many
> > places, and we end up with goofy code like this where one param takes a raw paddr,
> > and another uses page_to_phys().
> > 
> > @@ -1583,7 +1578,7 @@ u64 tdh_vp_addcx(struct tdx_vp *vp, struct page *tdcx_page)
> >  {
> >         struct tdx_module_args args = {
> >                 .rcx = page_to_phys(tdcx_page),
> > -               .rdx = tdx_tdvpr_pa(vp),
> > +               .rdx = vp->tdvpr_pa,
> >         };
> 
> I'm kinda dense normally and my coffee hasn't kicked in yet. What
> clearly does not work there?

Relying on struct page to provide type safety.

> Yeah, vp->tdvpr_pa is storing a physical address as a raw u64 and not a
> 'struct page'. That's not ideal. But it's also for a pretty good reason.

Right, but my point is that regradless of the justification, every exception to
passing a struct page diminishes the benefits of using struct page in the first
place.

> The "use 'struct page *' instead of u64 for physical addresses" thingy
> is a good pattern, not an absolute rule. Use it when you can, but
> abandon it for the greater good when necessary.
> 
> I don't hate the idea of a tdx_page_t. I'm just not sure it's worth the
> trouble. I'd certainly take a good look at the patches if someone hacked
> it together.