linux-kernel - Re: [PATCH V2 1/1] x86/tdx: Route safe halt execution via tdx_safe

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Message-ID: <CAGtprH-5bL44c7ZQHKsDuOQNNd4dsBd-uR8GT9OyqffEXW963Q@mail.gmail.com>
Date: Mon, 3 Feb 2025 09:01:41 -0800
From: Vishal Annapurve <vannapurve@...gle.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com, 
	seanjc@...gle.com, erdemaktas@...gle.com, ackerleytng@...gle.com, 
	jxgao@...gle.com, sagis@...gle.com, oupton@...gle.com, pgonda@...gle.com, 
	dave.hansen@...ux.intel.com, linux-coco@...ts.linux.dev, 
	chao.p.peng@...ux.intel.com, isaku.yamahata@...il.com, stable@...r.kernel.org
Subject: Re: [PATCH V2 1/1] x86/tdx: Route safe halt execution via tdx_safe_halt()

On Mon, Feb 3, 2025 at 8:00 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> ...
> >
> > Are you hinting towards a model where TDX guest prohibits such call
> > sites from being configured? I am not sure if it's a sustainable model
> > if we just rely on the host not advertising these features as the
> > guest kernel can still add new paths that are not controlled by the
> > host that lead to *_safe_halt().
>
> I've asked TDX module folks to provide additional information in ve_info
> to help handle STI shadow correctly. They will implement it, but it will
> take some time.

What will the final solution look like?

>
> So we need some kind of stopgap until we have it.

Does it make sense to carry the patch suggested by Sean [1] as a
stopgap for now?

[1] https://lore.kernel.org/lkml/Z5l6L3Hen9_Y3SGC@google.com/

>
> I am reluctant to commit to paravirt calls for this workaround. They will
> likely stick forever. It is possible, I would like to avoid them. If not,
> oh well.
>
> > > > 2) acpi_safe_halt() -> safe_halt() -> raw_safe_halt() -> arch_safe_halt()
> > >
> > > Have you checked why you get there? I don't see a reason for TDX guest to
> > > get into ACPI idle stuff. We don't have C-states to manage.
> >
> > Apparently userspace VMM is advertising pblock_address through SSDT
> > tables in my configuration which causes guests to enable ACPI cpuidle
> > drivers. Do you know if future generations of TDX hardware will not
> > support different c-states for TDX VMs?
>
> I have very limited understanding of power management, but I don't see how
> C-states can be meaningfully supported by any virtualized environment.
> To me, C-states only make sense for baremetal.

One possibility is that host can convey guests about using "mwait" as
cstate entry mechanism as an alternative to halt if supported.

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov