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

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Message-ID: <p6sbwtdmvwcbr55a4fmiirabkvp3f542nawdgxoyq22cdhnu33@ffbmyh2zuj2z>
Date: Thu, 30 Jan 2025 11:27:56 +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 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