On 04/21/2017 03:14 AM, Jan Beulich
wrote:
On 20.04.17 at 19:49, <mohit.gambhir@xxxxxxxxxx> wrote:
This patch changes wrmsrl() calls to write to MSR_P6_EVTSEL register in the
VPMU to wrmsr_safe(). There are known (and possibly some unknown) cases where
setting certain bits in MSR_P6_EVTSEL reg. can cause a General Protection
fault on some machines. Unless we catch this fault when it happens, it will
result in a hypervisor crash.
For instance, setting Pin Control (PC) bit (19) in MSR_P6_EVNTSEL results
in a General Protection Fault on Broadwell machines and thus causes the
hypervisor to crash.
I can't seem to find indication of this in the SDM. Is this removal of a
valid bit from an architectural MSR documented anywhere?
It see it as an observed hardware bug so I don't expect it to be
found in the SDM. I reached out to
the folks at Intel (CC) but they are unaware of it. Ideally it
should work the way it says in the SDM
for all events and on all machines but it doesn't - not for the all
the events and certainly not on all
machines.
Also, from the description in the SDM, PC flag bit it seems very
disruptive to me.
SDM says that if the bit is set then the processor toggles the PMi
pin (generating a performance monitoring interrupt?)
every time the event occurs. So if we program the counter to count
"unhaulted core cycles", and set PC flag bit
it will generate an interrupts every cycle!?
@@ -356,7 +356,9 @@ static inline void __core2_vpmu_load(struct vcpu *v)
for ( i = 0; i < arch_pmc_cnt; i++ )
{
wrmsrl(pmc_start + i, xen_pmu_cntr_pair[i].counter);
- wrmsrl(MSR_P6_EVNTSEL(i), xen_pmu_cntr_pair[i].control);
+ if ( wrmsr_safe(MSR_P6_EVNTSEL(i), xen_pmu_cntr_pair[i].control) ==
+ -EFAULT)
+ return -EFAULT;
If the function already returns an error code, please use it to avoid
a latent bug when in the future it may also return other than -EFAULT.
Fixed in next version.
@@ -369,6 +371,7 @@ static inline void __core2_vpmu_load(struct vcpu *v)
core2_vpmu_cxt->global_ovf_ctrl = 0;
wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, core2_vpmu_cxt->global_ctrl);
}
+ return 0;
Blank line above the main return statement of a function please.
pv_inject_hw_exception(TRAP_gp_fault
Fixed in next version.
@@ -461,9 +464,8 @@ static int core2_vpmu_load(struct vcpu *v, bool_t
from_guest)
vpmu_set(vpmu, VPMU_CONTEXT_LOADED);
- __core2_vpmu_load(v);
+ return __core2_vpmu_load(v);
- return 0;
}
Please also remove the now stray blank line.
Fixed in next version.
@@ -538,7 +540,8 @@ static int core2_vpmu_msr_common_check(u32 msr_index, int *type, int *index)
/* Do the lazy load staff. */
if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_LOADED) )
{
- __core2_vpmu_load(current);
+ if ( __core2_vpmu_load(current) )
+ return 0;
Nothing you need to change, but the change here point out that
the function really needs switching to returning bool.
Agreed.
@@ -719,8 +722,11 @@ static int core2_vpmu_do_wrmsr(unsigned int msr, uint64_t msr_content,
}
}
- if ( type != MSR_TYPE_GLOBAL )
- wrmsrl(msr, msr_content);
+ if ( type != MSR_TYPE_GLOBAL)
+ {
+ if ( wrmsr_safe(msr, msr_content) == -EFAULT )
+ return -EFAULT;
See the earlier similar comment.
Fixed in next verison.
Further a more general question: Why do you do this only for
MSR_P6_EVNTSEL*, but not consistently for all MSRs which are
being written with (partially) guest controlled values? I'd namely
expect all wrmsrl() to be switched over, unless it is clear that
we're dealing with an erratum here and we therefore can
assume that normal guarantees for architectural MSRs apply to
everything else.
I think it is an erratum for one specific MSR which is MSR_P6_EVTSEL
And finally, in the description you talk about forwarding the
fault - I can see this being the case for core2_vpmu_do_wrmsr(),
but how does this match up with __core2_vpmu_load() and its
callers? Namely you may report an error there after already
having updated some MSRs. Leaving the overall set of MSRs in
an inconsistent state is not very likely to be a good idea.
Yes, that's something I did not think about and needs to be fixed.
There are two issues here -
1. What should we set MSR values in case of an error? We can not
revert them because if it is part of a
context switch then we do not want to leak the values from the
previous guest to the new guest.
So one possible solution is to wipe all PMU MSRs (set them to 0)
before returning the error.
2. __core2_vpmu_load is called in two scenarios - One from a hyper
call XENPMU_FLUSH from do xenpmu_op()
and the other during a context switch from vpmu_switch_to(). The
hypercall seems to be handling the error
correctly but vpmu_switch_to() ignores the error. So to propagate
the error up to the guest kernel maybe we should
inject the fault into the guest using
hvm_inject_hw_exception(TRAP_gp_fault, error)/
pv_inject_hw_exception(TRAP_gp_fault, error)?
Jan
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