linux-kernel - Re: [RFC, PATCH 05/12] KVM: TDX: Add tdx_pamt

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Message-ID: <u6f6jbgbbz2judwuvwtelxdkhbl2dsqc2fqi2n4uvfwhszan75@2kvbsreeywrb>
Date: Thu, 8 May 2025 16:21:48 +0300
From: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Dave Hansen <dave.hansen@...el.com>, 
	"Huang, Kai" <kai.huang@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"seanjc@...gle.com" <seanjc@...gle.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, 
	"bp@...en8.de" <bp@...en8.de>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, 
	"x86@...nel.org" <x86@...nel.org>, "mingo@...hat.com" <mingo@...hat.com>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCH 05/12] KVM: TDX: Add tdx_pamt_get()/put() helpers

On Thu, May 08, 2025 at 10:08:32AM +0800, Yan Zhao wrote:
> On Wed, May 07, 2025 at 09:31:22AM -0700, Dave Hansen wrote:
> > On 5/5/25 05:44, Huang, Kai wrote:
> > >> +static int tdx_pamt_add(atomic_t *pamt_refcount, unsigned long hpa,
> > >> +			struct list_head *pamt_pages)
> > >> +{
> > >> +	u64 err;
> > >> +
> > >> +	hpa = ALIGN_DOWN(hpa, SZ_2M);
> > >> +
> > >> +	spin_lock(&pamt_lock);
> > > Just curious, Can the lock be per-2M-range?
> > 
> > Folks, please keep it simple.
> > 
> > If there's lock contention on this, we'll fix the lock contention, or
> > hash the physical address into a fixed number of locks. But having it be
> > per-2M-range sounds awful. Then you have to size it, and allocate it and
> > then resize it if there's ever hotplug, etc...
> In patch 2, there're per-2M-range pamt_refcounts. Could the per-2M-range
> lock be implemented in a similar way?
> 
> +static atomic_t *pamt_refcounts;
> +atomic_t *tdx_get_pamt_refcount(unsigned long hpa)
> +{
> +       return &pamt_refcounts[hpa / PMD_SIZE];
> +}

But why? If no contention, it is just wasteful.

> > Kirill, could you put together some kind of torture test for this,
> > please? I would imagine a workload which is sitting in a loop setting up
> > and tearing down VMs on a bunch of CPUs would do it.
> > 
> > That ^ would be the worst possible case, I think. If you don't see lock
> > contention there, you'll hopefully never see it on real systems.
> When one vCPU is trying to install a guest page of HPA A, while another vCPU
> is trying to install a guest page of HPA B, theoretically they may content the
> global pamt_lock even if HPA A and B belong to different PAMT 2M blocks.

This contention will be be momentary if ever happen.

> > I *suspect* that real systems will get bottlenecked somewhere in the
> > page conversion process rather than on this lock. But it should be a
> > pretty simple experiment to run.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov