Date:   Thu, 18 Nov 2021 00:23:29 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Andi Kleen <ak@linux.intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Juergen Gross <jgross@suse.com>, Deep Shah <sdeep@vmware.com>,
        "VMware, Inc." <pv-drivers@vmware.com>
Subject: Re: [PATCH] x86/paravirt: Fix build PARAVIRT_XXL=y without XEN_PV
Message-ID: <20211117232329.GD174703@worktop.programming.kicks-ass.net>
References: <20211117181439.4368-1-kirill.shutemov@linux.intel.com>
 <YZVLVfd5E6d6YQig@hirez.programming.kicks-ass.net>
 <20211117184225.6e257nfpdd2qhrj4@box.shutemov.name>
 <4824bf30-851e-c927-a50f-87fa2a429b2a@linux.intel.com>
 <YZVOfGtHyiZg1pIP@zn.tnic>
 <980ceab6-6686-c8f3-72b8-5743ca517bdf@linux.intel.com>
 <0f0b9784-1902-1526-2796-7d1a7ab17fb6@intel.com>
 <21f1325d-d97e-1bb7-ea87-d84e44089ab4@linux.intel.com>
 <YZVvdyHhPTzzZbiu@zn.tnic>
 <d826f932-a6a4-de7d-b0ea-f8e1f9bfe012@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d826f932-a6a4-de7d-b0ea-f8e1f9bfe012@linux.intel.com>
Precedence: bulk

On Wed, Nov 17, 2021 at 03:04:11PM -0800, Sathyanarayanan Kuppuswamy wrote:

> We need PV support to handle halt() and safe_halt() cases.
> 
> HLT instruction is generally used in cases like reboot, idle and
> exception fixup handlers.

Which exception calls hlt? Because idle and reboot can easily be done.

> In TDX guest, to support HLT instruction, it has to be emulated using
> a hypercall (aka TDVMCALL).
> 
> We have the following three ways to emulate the HLT instruction:
> 
> 1. Directly substitute TDVMCALLs in places where we require emulation.
> 2. Use #VE exception handler to emulate it (In TDX guest, executing HLT
>    will lead to #VE exception).
> 3. Emulate it using pv_ops
> 
> Since option#1 is not a scalable approach, it can be ignored. Option #2
> is also not preferred because, we cannot differentiate between safe
> halt and normal halt use cases in the exception handler. 

Would not regs->flags & IF provide clue? I know STI normally has a
shadow, but wouldn't a trap in that shadow still get the flag straight?
I'm sure there's fun bugs around this, but surely TDX is new and doesn't
have these bugs.