On Mon, May 4, 2015 at 8:02 AM, Boris Ostrovsky
<boris.ostrov...@oracle.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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