linux-kernel - Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters allocation for sparse memory

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Message-ID: <ca13c7f77f2d36fa12e25cf2b9fb61861c9ed38c.camel@intel.com>
Date: Wed, 1 Oct 2025 00:32:59 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Huang, Kai"
	<kai.huang@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "kas@...nel.org" <kas@...nel.org>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "Gao, Chao"
	<chao.gao@...el.com>, "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org"
	<x86@...nel.org>
Subject: Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters
 allocation for sparse memory

On Wed, 2025-09-24 at 16:57 +0800, Binbin Wu wrote:
> > This will results in @start pointing to the first refcount and @end
> > pointing to the second, IIUC.
> > 
> > So it seems we need:
> > 
> >       start = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(start_pfn));
> >       end   = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(end_pfn) - 1));
> >       start = round_down(start, PAGE_SIZE);
> >       end   = round_up(end, PAGE_SIZE);
> 
> Checked again, this seems to be the right version.

Thanks both for the analysis. I lazily created a test program to check some edge
cases and found the original and this version were both buggy. Clearly this code
needs to be clearer (as actually Dave pointed out in v2 and I failed to
address). Example (synthetic failure):

start_pfn = 0x80000
end_pfn =   0x80001

Original code: start = 0xff76ba4f9e034000
               end   = 0xff76ba4f9e034000

Above fix:     start = 0xff76ba4f9e034000
               end   = 0xff76ba4f9e034000

Part of the problem is that tdx_find_pamt_refcount() expects the hpa passed in
to be PMD aligned. The other callers of tdx_find_pamt_refcount() also make sure
that the PA passed in is 2MB aligned before calling, and compute this starting
with a PFN. So to try to make it easier to read and be correct what do you think
about the below:

static atomic_t *tdx_find_pamt_refcount(unsigned long pfn) {
    unsigned long hpa = ALIGN_DOWN(pfn, PMD_SIZE);

    return &pamt_refcounts[hpa / PMD_SIZE];
}

/*
 * 'start_pfn' is inclusive and 'end_pfn' is exclusive. Compute the 
 * page range to be inclusive of the start and end refcount
 * addresses and at least a page in size. The teardown logic needs
 * to handle potentially overlapping refcounts mappings resulting
 * from this.
 */
start = (unsigned long)tdx_find_pamt_refcount(start_pfn);
end   = (unsigned long)tdx_find_pamt_refcount(end_pfn - 1);
start = ALIGN_DOWN(start, PAGE_SIZE);
end   = ALIGN_DOWN(end, PAGE_SIZE) + PAGE_SIZE;