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

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Message-ID: <wmdg54v56uizuifhaufllnjtecmvhllv35jyrvdilf4ty4pfs5@y4zppjm2sthr>
Date: Thu, 30 Jan 2025 20:48:02 +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 09:24:37AM -0800, Vishal Annapurve wrote:
> On Thu, Jan 30, 2025 at 1:28 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
> >
> > On Wed, Jan 29, 2025 at 11:25:25PM +0000, Vishal Annapurve wrote:
> > > Direct HLT instruction execution causes #VEs for TDX VMs which is routed
> > > to hypervisor via tdvmcall. This process renders HLT instruction
> > > execution inatomic, so any preceding instructions like STI/MOV SS will
> > > end up enabling interrupts before the HLT instruction is routed to the
> > > hypervisor. This creates scenarios where interrupts could land during
> > > HLT instruction emulation without aborting halt operation leading to
> > > idefinite halt wait times.
> > >
> > > Commit bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests") already
> > > upgraded x86_idle() to invoke tdvmcall to avoid such scenarios, but
> > > it didn't cover pv_native_safe_halt() which can be invoked using
> > > raw_safe_halt() from call sites like acpi_safe_halt().
> > >
> > > raw_safe_halt() also returns with interrupts enabled so upgrade
> > > tdx_safe_halt() to enable interrupts by default and ensure that paravirt
> > > safe_halt() executions invoke tdx_safe_halt(). Earlier x86_idle() is now
> > > handled via tdx_idle() which simply invokes tdvmcall while preserving
> > > irq state.
> > >
> > > To avoid future call sites which cause HLT instruction emulation with
> > > irqs enabled, add a warn and fail the HLT instruction emulation.
> > >
> > > Cc: stable@...r.kernel.org
> > > Fixes: bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")
> > > Signed-off-by: Vishal Annapurve <vannapurve@...gle.com>
> > > ---
> > > Changes since V1:
> > > 1) Addressed comments from Dave H
> > >    - Comment regarding adding a check for TDX VMs in halt path is not
> > >      resolved in v2, would like feedback around better place to do so,
> > >      maybe in pv_native_safe_halt (?).
> > > 2) Added a new version of tdx_safe_halt() that will enable interrupts.
> > > 3) Previous tdx_safe_halt() implementation is moved to newly introduced
> > > tdx_idle().
> > >
> > > V1: https://lore.kernel.org/lkml/Z5l6L3Hen9_Y3SGC@google.com/T/
> > >
> > >  arch/x86/coco/tdx/tdx.c    | 23 ++++++++++++++++++++++-
> > >  arch/x86/include/asm/tdx.h |  2 +-
> > >  arch/x86/kernel/process.c  |  2 +-
> > >  3 files changed, 24 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > > index 0d9b090b4880..cc2a637dca15 100644
> > > --- a/arch/x86/coco/tdx/tdx.c
> > > +++ b/arch/x86/coco/tdx/tdx.c
> > > @@ -14,6 +14,7 @@
> > >  #include <asm/ia32.h>
> > >  #include <asm/insn.h>
> > >  #include <asm/insn-eval.h>
> > > +#include <asm/paravirt_types.h>
> > >  #include <asm/pgtable.h>
> > >  #include <asm/set_memory.h>
> > >  #include <asm/traps.h>
> > > @@ -380,13 +381,18 @@ static int handle_halt(struct ve_info *ve)
> > >  {
> > >       const bool irq_disabled = irqs_disabled();
> > >
> > > +     if (!irq_disabled) {
> > > +             WARN_ONCE(1, "HLT instruction emulation unsafe with irqs enabled\n");
> > > +             return -EIO;
> > > +     }
> > > +
> >
> > 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov