* [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead
@ 2020-05-05 6:16 Jan Beulich
2020-05-18 16:52 ` Roger Pau Monné
2020-05-21 16:46 ` Andrew Cooper
0 siblings, 2 replies; 5+ messages in thread
From: Jan Beulich @ 2020-05-05 6:16 UTC (permalink / raw)
To: xen-devel; +Cc: Andrew Cooper, Wei Liu, Roger Pau Monné
While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly
expensive (but still needed only for the toggle_guest_mode() path), the
effect of the latter on the exit-to-guest path is not insignificant.
Move the logic into toggle_guest_mode(), on the basis that
toggle_guest_pt() will always be invoked in pairs, yet we can't safely
undo the setting of root_pgt_changed during the second of these
invocations.
While at it, add a comment ahead of toggle_guest_pt() to clarify its
intended usage.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v3: Add comment ahead of toggle_guest_pt().
v2: Extend description.
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -393,18 +393,10 @@ bool __init xpti_pcid_enabled(void)
static void _toggle_guest_pt(struct vcpu *v)
{
- const struct domain *d = v->domain;
- struct cpu_info *cpu_info = get_cpu_info();
unsigned long cr3;
v->arch.flags ^= TF_kernel_mode;
update_cr3(v);
- if ( d->arch.pv.xpti )
- {
- cpu_info->root_pgt_changed = true;
- cpu_info->pv_cr3 = __pa(this_cpu(root_pgt)) |
- (d->arch.pv.pcid ? get_pcid_bits(v, true) : 0);
- }
/*
* Don't flush user global mappings from the TLB. Don't tick TLB clock.
@@ -412,15 +404,11 @@ static void _toggle_guest_pt(struct vcpu
* In shadow mode, though, update_cr3() may need to be accompanied by a
* TLB flush (for just the incoming PCID), as the top level page table may
* have changed behind our backs. To be on the safe side, suppress the
- * no-flush unconditionally in this case. The XPTI CR3 write, if enabled,
- * will then need to be a flushing one too.
+ * no-flush unconditionally in this case.
*/
cr3 = v->arch.cr3;
- if ( shadow_mode_enabled(d) )
- {
+ if ( shadow_mode_enabled(v->domain) )
cr3 &= ~X86_CR3_NOFLUSH;
- cpu_info->pv_cr3 &= ~X86_CR3_NOFLUSH;
- }
write_cr3(cr3);
if ( !(v->arch.flags & TF_kernel_mode) )
@@ -436,6 +424,8 @@ static void _toggle_guest_pt(struct vcpu
void toggle_guest_mode(struct vcpu *v)
{
+ const struct domain *d = v->domain;
+
ASSERT(!is_pv_32bit_vcpu(v));
/* %fs/%gs bases can only be stale if WR{FS,GS}BASE are usable. */
@@ -449,8 +439,27 @@ void toggle_guest_mode(struct vcpu *v)
asm volatile ( "swapgs" );
_toggle_guest_pt(v);
+
+ if ( d->arch.pv.xpti )
+ {
+ struct cpu_info *cpu_info = get_cpu_info();
+
+ cpu_info->root_pgt_changed = true;
+ cpu_info->pv_cr3 = __pa(this_cpu(root_pgt)) |
+ (d->arch.pv.pcid ? get_pcid_bits(v, true) : 0);
+ /*
+ * As in _toggle_guest_pt() the XPTI CR3 write needs to be a TLB-
+ * flushing one too for shadow mode guests.
+ */
+ if ( shadow_mode_enabled(d) )
+ cpu_info->pv_cr3 &= ~X86_CR3_NOFLUSH;
+ }
}
+/*
+ * Must be called in matching pairs without returning to guest context
+ * inbetween.
+ */
void toggle_guest_pt(struct vcpu *v)
{
if ( !is_pv_32bit_vcpu(v) )
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead
2020-05-05 6:16 [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead Jan Beulich
@ 2020-05-18 16:52 ` Roger Pau Monné
2020-05-21 16:46 ` Andrew Cooper
1 sibling, 0 replies; 5+ messages in thread
From: Roger Pau Monné @ 2020-05-18 16:52 UTC (permalink / raw)
To: Jan Beulich; +Cc: xen-devel, Wei Liu, Andrew Cooper
On Tue, May 05, 2020 at 08:16:03AM +0200, Jan Beulich wrote:
> While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly
> expensive (but still needed only for the toggle_guest_mode() path), the
> effect of the latter on the exit-to-guest path is not insignificant.
> Move the logic into toggle_guest_mode(), on the basis that
> toggle_guest_pt() will always be invoked in pairs, yet we can't safely
> undo the setting of root_pgt_changed during the second of these
> invocations.
I'm not sure if it would be worth to add a comment to note the
intended usage of toggle_guest_pt is to fetch data from the kernel
page tables when running in user mode. The one about using it in pairs
is certainly fine.
> While at it, add a comment ahead of toggle_guest_pt() to clarify its
> intended usage.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Thanks, Roger.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead
2020-05-05 6:16 [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead Jan Beulich
2020-05-18 16:52 ` Roger Pau Monné
@ 2020-05-21 16:46 ` Andrew Cooper
2020-05-22 10:07 ` Jan Beulich
1 sibling, 1 reply; 5+ messages in thread
From: Andrew Cooper @ 2020-05-21 16:46 UTC (permalink / raw)
To: Jan Beulich, xen-devel; +Cc: Wei Liu, Roger Pau Monné
On 05/05/2020 07:16, Jan Beulich wrote:
> While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly
> expensive (but still needed only for the toggle_guest_mode() path), the
> effect of the latter on the exit-to-guest path is not insignificant.
> Move the logic into toggle_guest_mode(), on the basis that
> toggle_guest_pt() will always be invoked in pairs, yet we can't safely
> undo the setting of root_pgt_changed during the second of these
> invocations.
>
> While at it, add a comment ahead of toggle_guest_pt() to clarify its
> intended usage.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
I'm still of the opinion that the commit message wants rewriting to get
the important points across clearly.
And those are that toggle_guest_pt() is called in pairs specifically to
read kernel data structures when emulating a userspace action, and that
this doesn't modify cr3 from the guests point of view, and therefore
doesn't need the resync on exit-to-guest path.
~Andrew
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead
2020-05-21 16:46 ` Andrew Cooper
@ 2020-05-22 10:07 ` Jan Beulich
2020-05-29 16:24 ` Andrew Cooper
0 siblings, 1 reply; 5+ messages in thread
From: Jan Beulich @ 2020-05-22 10:07 UTC (permalink / raw)
To: Andrew Cooper, xen-devel; +Cc: Wei Liu, Roger Pau Monné
On 21.05.2020 18:46, Andrew Cooper wrote:
> On 05/05/2020 07:16, Jan Beulich wrote:
>> While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly
>> expensive (but still needed only for the toggle_guest_mode() path), the
>> effect of the latter on the exit-to-guest path is not insignificant.
>> Move the logic into toggle_guest_mode(), on the basis that
>> toggle_guest_pt() will always be invoked in pairs, yet we can't safely
>> undo the setting of root_pgt_changed during the second of these
>> invocations.
>>
>> While at it, add a comment ahead of toggle_guest_pt() to clarify its
>> intended usage.
>>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> I'm still of the opinion that the commit message wants rewriting to get
> the important points across clearly.
>
> And those are that toggle_guest_pt() is called in pairs specifically to
> read kernel data structures when emulating a userspace action, and that
> this doesn't modify cr3 from the guests point of view, and therefore
> doesn't need the resync on exit-to-guest path.
Is this
"toggle_guest_pt() is called in pairs, to read guest kernel data
structures when emulating a guest userspace action. Hence this doesn't
modify cr3 from the guest's point of view, and therefore doesn't need
any resync on the exit-to-guest path. Therefore move the updating of
->pv_cr3 and ->root_pgt_changed into toggle_guest_mode(), since undoing
the changes during the second of these invocations wouldn't be a safe
thing to do."
any better?
Jan
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead
2020-05-22 10:07 ` Jan Beulich
@ 2020-05-29 16:24 ` Andrew Cooper
0 siblings, 0 replies; 5+ messages in thread
From: Andrew Cooper @ 2020-05-29 16:24 UTC (permalink / raw)
To: Jan Beulich, xen-devel; +Cc: Wei Liu, Roger Pau Monné
On 22/05/2020 11:07, Jan Beulich wrote:
> On 21.05.2020 18:46, Andrew Cooper wrote:
>> On 05/05/2020 07:16, Jan Beulich wrote:
>>> While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly
>>> expensive (but still needed only for the toggle_guest_mode() path), the
>>> effect of the latter on the exit-to-guest path is not insignificant.
>>> Move the logic into toggle_guest_mode(), on the basis that
>>> toggle_guest_pt() will always be invoked in pairs, yet we can't safely
>>> undo the setting of root_pgt_changed during the second of these
>>> invocations.
>>>
>>> While at it, add a comment ahead of toggle_guest_pt() to clarify its
>>> intended usage.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> I'm still of the opinion that the commit message wants rewriting to get
>> the important points across clearly.
>>
>> And those are that toggle_guest_pt() is called in pairs specifically to
>> read kernel data structures when emulating a userspace action, and that
>> this doesn't modify cr3 from the guests point of view, and therefore
>> doesn't need the resync on exit-to-guest path.
> Is this
>
> "toggle_guest_pt() is called in pairs, to read guest kernel data
> structures when emulating a guest userspace action. Hence this doesn't
> modify cr3 from the guest's point of view, and therefore doesn't need
> any resync on the exit-to-guest path. Therefore move the updating of
> ->pv_cr3 and ->root_pgt_changed into toggle_guest_mode(), since undoing
> the changes during the second of these invocations wouldn't be a safe
> thing to do."
>
> any better?
Yes - that will do.
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
^ permalink raw reply [flat|nested] 5+ messages in thread
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2020-05-05 6:16 [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead Jan Beulich
2020-05-18 16:52 ` Roger Pau Monné
2020-05-21 16:46 ` Andrew Cooper
2020-05-22 10:07 ` Jan Beulich
2020-05-29 16:24 ` Andrew Cooper
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