linux-kernel - Re: [PATCH v8 07/11] x86/tdx: Add HLT support for TDX guest

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Message-ID: <20211006191716.pyhwbgfyqugoupmg@treble>
Date:   Wed, 6 Oct 2021 12:17:16 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>,
        VMware Inc <pv-drivers@...are.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 07/11] x86/tdx: Add HLT support for TDX guest

On Mon, Oct 04, 2021 at 07:52:01PM -0700, Kuppuswamy Sathyanarayanan wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> +	 * Emulate HLT operation via hypercall. More info about ABI
> +	 * can be found in TDX Guest-Host-Communication Interface
> +	 * (GHCI), sec 3.8 TDG.VP.VMCALL<Instruction.HLT>.
> +	 *
> +	 * The VMM uses the "IRQ disabled" param to understand IRQ
> +	 * enabled status (RFLAGS.IF) of TD guest and determine
> +	 * whether or not it should schedule the halted vCPU if an
> +	 * IRQ becomes pending. E.g. if IRQs are disabled the VMM
> +	 * can keep the vCPU in virtual HLT, even if an IRQ is
> +	 * pending, without hanging/breaking the guest.
> +	 *
> +	 * do_sti parameter is used by __tdx_hypercall() to decide
> +	 * whether to call STI instruction before executing TDCALL
> +	 * instruction.
> +	 */
> +	ret = _tdx_hypercall(EXIT_REASON_HLT, irq_disabled, 0, 0, do_sti, NULL);
> +
> +	/*
> +	 * Use WARN_ONCE() to report the failure. Since tdx_*halt() calls
> +	 * are also used in pv_ops, #VE error handler cannot be used to
> +	 * report the failure.
> +	 */
> +	WARN_ONCE(ret, "HLT instruction emulation failed\n");

I'm having trouble following this last comment, does it mean there's no
way to return the error back to the #VE handler when this is called in
#VE context?  Seems like that would be any easy problem to solve by
shuffling the functions a bit.

-- 
Josh