From mboxrd@z Thu Jan  1 00:00:00 1970
From: Keir Fraser <keir.xen@gmail.com>
Subject: Re: [PATCH] libxc: set CPUID topology leaf as unsupported
	for PV guests
Date: Thu, 14 Jul 2011 14:30:28 +0100
Message-ID: <CA44AE04.1DDBE%keir.xen@gmail.com>
References: <b3705aae543b95fd7f6b.1310644219@qabil>
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To: David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xensource.com
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On 14/07/2011 12:50, "David Vrabel" <david.vrabel@citrix.com> wrote:

> # HG changeset patch
> # User David Vrabel <david.vrabel@citrix.com>
> # Date 1310643124 -3600
> # Branch cpuid
> # Node ID b3705aae543b95fd7f6bb38c21b5f1edd2b70609
> # Parent  98701b1276c034b2bbbc8c7a975cf4c361caaa63
> libxc: set CPUID topology leaf as unsupported for PV guests
> 
> The result of a CPUID Extended Topology Enumeration leaf for PV guests
> is invalid as the level in ECX is ignored.  This can cause some guests
> to loop endlessly when trying to enumerate the topology.
> 
> Since the physical topology isn't useful to PV guests set the topology
> leaf as unsupported.

For future reference, you also need to modify
xen/arch/x86/traps.c:pv_cpuid(), for dom0.

 -- Keir

> Guests affected include Linux kernels prior 2.6.32 where a workaround
> was applied ("xen: mask extended topology info in cpu",
> 82d6469916c6fcfa345636a49004c9d1753905d1).
> 
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> 
> diff -r 98701b1276c0 -r b3705aae543b tools/libxc/xc_cpuid_x86.c
> --- a/tools/libxc/xc_cpuid_x86.c Thu Jul 14 09:43:37 2011 +0100
> +++ b/tools/libxc/xc_cpuid_x86.c Thu Jul 14 12:32:04 2011 +0100
> @@ -487,6 +487,11 @@ static void xc_cpuid_pv_policy(
>          regs[0] = regs[2] = regs[3] = 0;
>          break;
>  
> +    case 0x0000000b:
> +        /* Extended Topology Enumeration leaf is unsupported. */
> +        regs[0] = regs[1] = regs[2] = regs[3] = 0;
> +        break;
> +
>      case 0x0000000d:
>          xc_cpuid_config_xsave(xch, domid, xfeature_mask, input, regs);
>          break;
> 
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