linux-kernel - Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap

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Message-ID: <8670cd6065b428c891a7d008500934a57f09b99f.camel@intel.com>
Date: Thu, 28 Aug 2025 21:33:48 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "michael.roth@....com" <michael.roth@....com>,
	"Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap_cnt()
 into its sole caller

On Thu, 2025-08-28 at 13:26 -0700, Sean Christopherson wrote:
> Me confused.  This is pre-boot, not the normal fault path, i.e. blocking other
> operations is not a concern.

Just was my recollection of the discussion. I found it:
https://lore.kernel.org/lkml/Zbrj5WKVgMsUFDtb@google.com/

> 
> If tdh_mr_extend() is too heavy for a non-preemptible section, then the current
> code is also broken in the sense that there are no cond_resched() calls.  The
> vast majority of TDX hosts will be using non-preemptible kernels, so without an
> explicit cond_resched(), there's no practical difference between extending the
> measurement under mmu_lock versus outside of mmu_lock.
> 
> _If_ we need/want to do tdh_mr_extend() outside of mmu_lock, we can and should
> still do tdh_mem_page_add() under mmu_lock.

I just did a quick test and we should be on the order of <1 ms per page for the
full loop. I can try to get some more formal test data if it matters. But that
doesn't sound too horrible?

tdh_mr_extend() outside MMU lock is tempting because it doesn't *need* to be
inside it. But maybe a better reason is that we could better handle errors
outside the fault. (i.e. no 5 line comment about why not to return an error in
tdx_mem_page_add() due to code in another file).

I wonder if Yan can give an analysis of any zapping races if we do that.