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

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Message-ID: <2wooixyr7ekw3ebi4oytuolk5wtyi2gqhsiveshfcfixlz3kuq@d5h6gniewqzk>
Date: Fri, 31 Jan 2025 10:13:24 +0200
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Vishal Annapurve <vannapurve@...gle.com>
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 Thu, Jan 30, 2025 at 11:45:01AM -0800, Vishal Annapurve wrote:
> On Thu, Jan 30, 2025 at 10:48 AM Kirill A. Shutemov
> <kirill@...temov.name> wrote:
> > ...
> > > >
> > > > I think it is worth to putting this into a separate patch and not
> > > > backport. The rest of the patch is bugfix and this doesn't belong.
> > > >
> > > > Otherwise, looks good to me:
> > > >
> > > > Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>@linux.intel.com>
> > > >
> > > > --
> > > >   Kiryl Shutsemau / Kirill A. Shutemov
> > >
> > > Thanks Kirill for the review.
> > >
> > > Thinking more about this fix, now I am wondering why the efforts [1]
> > > to move halt/safe_halt under CONFIG_PARAVIRT were abandoned. Currently
> > > proposed fix is incomplete as it would not handle scenarios where
> > > CONFIG_PARAVIRT_XXL is disabled. I am tilting towards reviving [1] and
> > > requiring CONFIG_PARAVIRT for TDX VMs. WDYT?
> > >
> > > [1] https://lore.kernel.org/lkml/20210517235008.257241-1-sathyanarayanan.kuppuswamy@linux.intel.com/
> >
> > Many people dislike paravirt callbacks. We tried to avoid relying on them
> > for core TDX enabling.
> >
> > Can you explain the issue you see with CONFIG_PARAVIRT_XXL being disabled?
> > I don't think I follow.
> 
> Relevant callers of *_safe_halt() are:
> 1) kvm_wait() -> safe_halt() -> raw_safe_halt() -> arch_safe_halt()

Okay, I didn't realized that CONFIG_PARAVIRT_SPINLOCKS doesn't depend on
CONFIG_PARAVIRT_XXL.

It would be interesting to check if paravirtualized spinlocks make sense
for TDX given the cost of TD exit.

Maybe we should avoid advertising KVM_FEATURE_PV_UNHALT to the TDX guests?

> 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov