linux-kernel - Re: [PATCH] x86/xen: avoid warning in Xen pv guest with CONFIG_AMD_MEM

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Date:   Mon, 25 Jan 2021 17:26:39 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Juergen Gross <jgross@...e.com>, <xen-devel@...ts.xenproject.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/xen: avoid warning in Xen pv guest with
 CONFIG_AMD_MEM_ENCRYPT enabled

On 25/01/2021 14:00, Juergen Gross wrote:
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 4409306364dc..82948251f57b 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -583,6 +583,14 @@ DEFINE_IDTENTRY_RAW(xenpv_exc_debug)
>  		exc_debug(regs);
>  }
>  
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +DEFINE_IDTENTRY_RAW(xenpv_exc_vmm_communication)
> +{
> +	/* This should never happen and there is no way to handle it. */
> +	panic("X86_TRAP_VC in Xen PV mode.");

Honestly, exactly the same is true of #VE, #HV and #SX.

What we do in the hypervisor is wire up one handler for all unknown
exceptions (to avoid potential future #DF issues) leading to a panic. 
Wouldn't it be better to do this unconditionally, especially as #GP/#NP
doesn't work for PV guests for unregistered callbacks, rather than
fixing up piecewise like this?

~Andrew