[PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Boris Ostrovsky]

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).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
---
v2: 
 * Adjusted ifdefs to account for !CONFIG_PVHVM case
 * Renamed xen_guest_init() back to xen_hvm_guest_init()

 arch/x86/include/asm/hypervisor.h |  2 +-
 arch/x86/kernel/cpu/hypervisor.c  |  4 ++--
 arch/x86/xen/enlighten.c          | 28 +++++++++++++++++++---------
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/hypervisor.h b/arch/x86/include/asm/hypervisor.h
index e42f758..055ea99 100644
--- a/arch/x86/include/asm/hypervisor.h
+++ b/arch/x86/include/asm/hypervisor.h
@@ -50,7 +50,7 @@ extern const struct hypervisor_x86 *x86_hyper;
 /* Recognized hypervisors */
 extern const struct hypervisor_x86 x86_hyper_vmware;
 extern const struct hypervisor_x86 x86_hyper_ms_hyperv;
-extern const struct hypervisor_x86 x86_hyper_xen_hvm;
+extern const struct hypervisor_x86 x86_hyper_xen;
 extern const struct hypervisor_x86 x86_hyper_kvm;
 
 extern void init_hypervisor(struct cpuinfo_x86 *c);
diff --git a/arch/x86/kernel/cpu/hypervisor.c b/arch/x86/kernel/cpu/hypervisor.c
index 36ce402..d820d8e 100644
--- a/arch/x86/kernel/cpu/hypervisor.c
+++ b/arch/x86/kernel/cpu/hypervisor.c
@@ -27,8 +27,8 @@
 
 static const __initconst struct hypervisor_x86 * const hypervisors[] =
 {
-#ifdef CONFIG_XEN_PVHVM
-	&x86_hyper_xen_hvm,
+#ifdef CONFIG_XEN
+	&x86_hyper_xen,
 #endif
 	&x86_hyper_vmware,
 	&x86_hyper_ms_hyperv,
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 94578ef..ac3427a 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1760,6 +1760,9 @@ static struct notifier_block xen_hvm_cpu_notifier = {
 
 static void __init xen_hvm_guest_init(void)
 {
+	if (xen_pv_domain())
+		return;
+
 	init_hvm_pv_info();
 
 	xen_hvm_init_shared_info();
@@ -1775,6 +1778,7 @@ static void __init xen_hvm_guest_init(void)
 	xen_hvm_init_time_ops();
 	xen_hvm_init_mmu_ops();
 }
+#endif
 
 static bool xen_nopv = false;
 static __init int xen_parse_nopv(char *arg)
@@ -1784,14 +1788,11 @@ static __init int xen_parse_nopv(char *arg)
 }
 early_param("xen_nopv", xen_parse_nopv);
 
-static uint32_t __init xen_hvm_platform(void)
+static uint32_t __init xen_platform(void)
 {
 	if (xen_nopv)
 		return 0;
 
-	if (xen_pv_domain())
-		return 0;
-
 	return xen_cpuid_base();
 }
 
@@ -1809,11 +1810,20 @@ bool xen_hvm_need_lapic(void)
 }
 EXPORT_SYMBOL_GPL(xen_hvm_need_lapic);
 
-const struct hypervisor_x86 x86_hyper_xen_hvm __refconst = {
-	.name			= "Xen HVM",
-	.detect			= xen_hvm_platform,
+static void xen_set_cpu_features(struct cpuinfo_x86 *c)
+{
+	if (xen_pv_domain())
+		clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
+}
+
+const struct hypervisor_x86 x86_hyper_xen = {
+	.name			= "Xen",
+	.detect			= xen_platform,
+#ifdef CONFIG_XEN_PVHVM
 	.init_platform		= xen_hvm_guest_init,
+#endif
 	.x2apic_available	= xen_x2apic_para_available,
+	.set_cpu_features       = xen_set_cpu_features,
 };
-EXPORT_SYMBOL(x86_hyper_xen_hvm);
-#endif
+EXPORT_SYMBOL(x86_hyper_xen);
+
-- 
1.9.3

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Andy Lutomirski]

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

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Andy Lutomirski]

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

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Boris Ostrovsky]

On 05/05/2015 09:51 AM, Andy Lutomirski wrote:
> On Mon, May 4, 2015 at 8:02 AM, Boris Ostrovsky
> <boris.ostrovsky@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?

No, this is meant for "true" PV guest not to go through HVM-specific 
initialization.

We have to do it since now x86_hyper_xen_hvm (renamed to x86_hyper_xen) 
will have init_platform() op and so init_hypervisor_platform() will call 
it obviously without knowing the type of the guest.

>
>> +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.

HVM guests execute SYSRET and they will continue having 
X86_BUG_SYSRET_SS_ATTRS flag on. PV guests OTOH don't do SYSRET, the 
hypervisor does (which is why if this issue is to be fixed it would have 
to be done in the hypervisor).

-boris

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Boris Ostrovsky]

On 05/05/2015 09:51 AM, Andy Lutomirski wrote:
> On Mon, May 4, 2015 at 8:02 AM, Boris Ostrovsky
> <boris.ostrovsky@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?

No, this is meant for "true" PV guest not to go through HVM-specific 
initialization.

We have to do it since now x86_hyper_xen_hvm (renamed to x86_hyper_xen) 
will have init_platform() op and so init_hypervisor_platform() will call 
it obviously without knowing the type of the guest.

>
>> +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.

HVM guests execute SYSRET and they will continue having 
X86_BUG_SYSRET_SS_ATTRS flag on. PV guests OTOH don't do SYSRET, the 
hypervisor does (which is why if this issue is to be fixed it would have 
to be done in the hypervisor).

-boris

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[David Vrabel]

On 04/05/15 16:02, Boris Ostrovsky wrote:
> 
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1760,6 +1760,9 @@ static struct notifier_block xen_hvm_cpu_notifier = {
>  
>  static void __init xen_hvm_guest_init(void)
>  {
> +	if (xen_pv_domain())
> +		return;
> +

I take Andy's point that this looks a little odd, perhaps...

static void __init xen_platform_init(void)
{
    if (!xen_pv_domain())
        xen_hvm_guest_init();
}

Would be a useful follow up?

I'll apply as-is though

David

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[David Vrabel]

On 04/05/15 16:02, Boris Ostrovsky wrote:
> 
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1760,6 +1760,9 @@ static struct notifier_block xen_hvm_cpu_notifier = {
>  
>  static void __init xen_hvm_guest_init(void)
>  {
> +	if (xen_pv_domain())
> +		return;
> +

I take Andy's point that this looks a little odd, perhaps...

static void __init xen_platform_init(void)
{
    if (!xen_pv_domain())
        xen_hvm_guest_init();
}

Would be a useful follow up?

I'll apply as-is though

David

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Boris Ostrovsky]

On 05/05/2015 01:25 PM, David Vrabel wrote:
> On 04/05/15 16:02, Boris Ostrovsky wrote:
>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -1760,6 +1760,9 @@ static struct notifier_block xen_hvm_cpu_notifier = {
>>   
>>   static void __init xen_hvm_guest_init(void)
>>   {
>> +	if (xen_pv_domain())
>> +		return;
>> +
> I take Andy's point that this looks a little odd, perhaps...
>
> static void __init xen_platform_init(void)
> {
>      if (!xen_pv_domain())
>          xen_hvm_guest_init();
> }
>
> Would be a useful follow up?
>
> I'll apply as-is though
>


Yes, this would look better. I can respin this if you want to avoid the 
extra patch.

-boris

Re: [PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Boris Ostrovsky]

On 05/05/2015 01:25 PM, David Vrabel wrote:
> On 04/05/15 16:02, Boris Ostrovsky wrote:
>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -1760,6 +1760,9 @@ static struct notifier_block xen_hvm_cpu_notifier = {
>>   
>>   static void __init xen_hvm_guest_init(void)
>>   {
>> +	if (xen_pv_domain())
>> +		return;
>> +
> I take Andy's point that this looks a little odd, perhaps...
>
> static void __init xen_platform_init(void)
> {
>      if (!xen_pv_domain())
>          xen_hvm_guest_init();
> }
>
> Would be a useful follow up?
>
> I'll apply as-is though
>


Yes, this would look better. I can respin this if you want to avoid the 
extra patch.

-boris

[PATCH v2] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Boris Ostrovsky]

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).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
---
v2: 
 * Adjusted ifdefs to account for !CONFIG_PVHVM case
 * Renamed xen_guest_init() back to xen_hvm_guest_init()

 arch/x86/include/asm/hypervisor.h |  2 +-
 arch/x86/kernel/cpu/hypervisor.c  |  4 ++--
 arch/x86/xen/enlighten.c          | 28 +++++++++++++++++++---------
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/hypervisor.h b/arch/x86/include/asm/hypervisor.h
index e42f758..055ea99 100644
--- a/arch/x86/include/asm/hypervisor.h
+++ b/arch/x86/include/asm/hypervisor.h
@@ -50,7 +50,7 @@ extern const struct hypervisor_x86 *x86_hyper;
 /* Recognized hypervisors */
 extern const struct hypervisor_x86 x86_hyper_vmware;
 extern const struct hypervisor_x86 x86_hyper_ms_hyperv;
-extern const struct hypervisor_x86 x86_hyper_xen_hvm;
+extern const struct hypervisor_x86 x86_hyper_xen;
 extern const struct hypervisor_x86 x86_hyper_kvm;
 
 extern void init_hypervisor(struct cpuinfo_x86 *c);
diff --git a/arch/x86/kernel/cpu/hypervisor.c b/arch/x86/kernel/cpu/hypervisor.c
index 36ce402..d820d8e 100644
--- a/arch/x86/kernel/cpu/hypervisor.c
+++ b/arch/x86/kernel/cpu/hypervisor.c
@@ -27,8 +27,8 @@
 
 static const __initconst struct hypervisor_x86 * const hypervisors[] =
 {
-#ifdef CONFIG_XEN_PVHVM
-	&x86_hyper_xen_hvm,
+#ifdef CONFIG_XEN
+	&x86_hyper_xen,
 #endif
 	&x86_hyper_vmware,
 	&x86_hyper_ms_hyperv,
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 94578ef..ac3427a 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1760,6 +1760,9 @@ static struct notifier_block xen_hvm_cpu_notifier = {
 
 static void __init xen_hvm_guest_init(void)
 {
+	if (xen_pv_domain())
+		return;
+
 	init_hvm_pv_info();
 
 	xen_hvm_init_shared_info();
@@ -1775,6 +1778,7 @@ static void __init xen_hvm_guest_init(void)
 	xen_hvm_init_time_ops();
 	xen_hvm_init_mmu_ops();
 }
+#endif
 
 static bool xen_nopv = false;
 static __init int xen_parse_nopv(char *arg)
@@ -1784,14 +1788,11 @@ static __init int xen_parse_nopv(char *arg)
 }
 early_param("xen_nopv", xen_parse_nopv);
 
-static uint32_t __init xen_hvm_platform(void)
+static uint32_t __init xen_platform(void)
 {
 	if (xen_nopv)
 		return 0;
 
-	if (xen_pv_domain())
-		return 0;
-
 	return xen_cpuid_base();
 }
 
@@ -1809,11 +1810,20 @@ bool xen_hvm_need_lapic(void)
 }
 EXPORT_SYMBOL_GPL(xen_hvm_need_lapic);
 
-const struct hypervisor_x86 x86_hyper_xen_hvm __refconst = {
-	.name			= "Xen HVM",
-	.detect			= xen_hvm_platform,
+static void xen_set_cpu_features(struct cpuinfo_x86 *c)
+{
+	if (xen_pv_domain())
+		clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
+}
+
+const struct hypervisor_x86 x86_hyper_xen = {
+	.name			= "Xen",
+	.detect			= xen_platform,
+#ifdef CONFIG_XEN_PVHVM
 	.init_platform		= xen_hvm_guest_init,
+#endif
 	.x2apic_available	= xen_x2apic_para_available,
+	.set_cpu_features       = xen_set_cpu_features,
 };
-EXPORT_SYMBOL(x86_hyper_xen_hvm);
-#endif
+EXPORT_SYMBOL(x86_hyper_xen);
+
-- 
1.9.3