linux-kernel - Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

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Message-ID: <diqzms9n4l8i.fsf@ackerleytng-ctop.c.googlers.com>
Date: Tue, 01 Jul 2025 15:09:01 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>, Vishal Annapurve <vannapurve@...gle.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, 
	"quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, 
	"Shutemov, Kirill" <kirill.shutemov@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>, 
	"david@...hat.com" <david@...hat.com>, "thomas.lendacky@....com" <thomas.lendacky@....com>, 
	"vbabka@...e.cz" <vbabka@...e.cz>, "tabba@...gle.com" <tabba@...gle.com>, "Du, Fan" <fan.du@...el.com>, 
	"michael.roth@....com" <michael.roth@....com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Peng, Chao P" <chao.p.peng@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Weiny, Ira" <ira.weiny@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, 
	"jroedel@...e.de" <jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>, 
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

Yan Zhao <yan.y.zhao@...el.com> writes:

> On Mon, Jun 30, 2025 at 10:22:26PM -0700, Vishal Annapurve wrote:
>> On Mon, Jun 30, 2025 at 10:04 PM Yan Zhao <yan.y.zhao@...el.com> wrote:
>> >
>> > On Tue, Jul 01, 2025 at 05:45:54AM +0800, Edgecombe, Rick P wrote:
>> > > On Mon, 2025-06-30 at 12:25 -0700, Ackerley Tng wrote:
>> > > > > So for this we can do something similar. Have the arch/x86 side of TDX grow
>> > > > > a
>> > > > > new tdx_buggy_shutdown(). Have it do an all-cpu IPI to kick CPUs out of
>> > > > > SEAMMODE, wbivnd, and set a "no more seamcalls" bool. Then any SEAMCALLs
>> > > > > after
>> > > > > that will return a TDX_BUGGY_SHUTDOWN error, or similar. All TDs in the
>> > > > > system
>> > > > > die. Zap/cleanup paths return success in the buggy shutdown case.
>> > > > >
>> > > >
>> > > > Do you mean that on unmap/split failure:
>> > >
>> > > Maybe Yan can clarify here. I thought the HWpoison scenario was about TDX module
>> > My thinking is to set HWPoison to private pages whenever KVM_BUG_ON() was hit in
>> > TDX. i.e., when the page is still mapped in S-EPT but the TD is bugged on and
>> > about to tear down.
>> >
>> > So, it could be due to KVM or TDX module bugs, which retries can't help.
>> >
>> > > bugs. Not TDX busy errors, demote failures, etc. If there are "normal" failures,
>> > > like the ones that can be fixed with retries, then I think HWPoison is not a
>> > > good option though.
>> > >
>> > > >  there is a way to make 100%
>> > > > sure all memory becomes re-usable by the rest of the host, using
>> > > > tdx_buggy_shutdown(), wbinvd, etc?
>> >
>> > Not sure about this approach. When TDX module is buggy and the page is still
>> > accessible to guest as private pages, even with no-more SEAMCALLs flag, is it
>> > safe enough for guest_memfd/hugetlb to re-assign the page to allow simultaneous
>> > access in shared memory with potential private access from TD or TDX module?
>> 
>> If no more seamcalls are allowed and all cpus are made to exit SEAM
>> mode then how can there be potential private access from TD or TDX
>> module?
> Not sure. As Kirill said "TDX module has creative ways to corrupt it"
> https://lore.kernel.org/all/zlxgzuoqwrbuf54wfqycnuxzxz2yduqtsjinr5uq4ss7iuk2rt@qaaolzwsy6ki/.
>
> Or, could TDX just set a page flag, like what for XEN
>
>         /* XEN */
>         /* Pinned in Xen as a read-only pagetable page. */
>         PG_pinned = PG_owner_priv_1,
>
> e.g.
> 	PG_tdx_firmware_access = PG_owner_priv_1,
>
> Then, guest_memfd checks this flag on every zap and replace it with PG_hwpoison
> on behalf of TDX?

I think this question probably arose because of a misunderstanding I
might have caused. I meant to set the HWpoison flag from the kernel, not
from within the TDX module. Please see [1].

In addition, if the TDX module (now referring specifically to the TDX
module and not the kernel) sets page flags, that won't work with
vmemmap-optimized folios. Setting a page flag on a vmemmap-optimized
folio will be setting the flag on a few pages.

[1] https://lore.kernel.org/all/diqzplej4llh.fsf@ackerleytng-ctop.c.googlers.com/