linux-kernel - Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS

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Date:	Tue, 5 May 2015 06:51:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	linux@...elenboom.it
Subject: Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on
 Xen PV guests

On Mon, May 4, 2015 at 8:02 AM, Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
> Commit 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor
> attribute issue") makes AMD processors set SS to __KERNEL_DS in
> __switch_to() to deal with cases when SS is NULL.
>
> This breaks Xen PV guests who do not want to load SS with__KERNEL_DS.
>
> Since the problem that the commit is trying to address would have to be
> fixed in the hypervisor (if it in fact exists under Xen) there is no
> reason to set X86_BUG_SYSRET_SS_ATTRS flag for PV VPCUs here.
>
> This can be easily achieved by adding x86_hyper_xen_hvm.set_cpu_features
> op which will clear this flag. (And since this structure is no longer
> HVM-specific we should do some renaming).

Looks good to me, but:

>  static void __init xen_hvm_guest_init(void)
>  {
> +       if (xen_pv_domain())
> +               return;
> +

How can a platform be "hvm" and "pv"?  Is that the PVHVM thing?

> +static void xen_set_cpu_features(struct cpuinfo_x86 *c)
> +{
> +       if (xen_pv_domain())
> +               clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
> +}

I haven't followed all the twists and turns, but, if the guest
platform is such that the guest kernel is really executing SYSRET and
we're on AMD, then we have the sysret_ss_attrs bug.  If PVHVM has
xen_pv_domain() returning true but also uses SYSRET for real, then
this looks wrong.

--Andy
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