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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aLFDeJ8Rk0p8yucV@yzhao56-desk.sh.intel.com>
Date: Fri, 29 Aug 2025 14:06:48 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "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>, "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 Fri, Aug 29, 2025 at 05:33:48AM +0800, Edgecombe, Rick P wrote:
> 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.
I actually proposed to have write mmu_lock around tdh_mem_page_add() and
tdh_mr_extend(), as in
https://lore.kernel.org/kvm/Ztfn5gh5888PmEIe@yzhao56-desk.sh.intel.com.

I don't see any reason why tdh_mr_extend() can't be done inside mmu_lock
in the pre-boot stage.

But the previous conclusion was that with slots_lock and filemap invalidation
lock, it's ok to invoke tdh_mem_page_add() and tdh_mr_extend() without any
mmu_lock. The nr_premapped can also detect the unexpected zap.