RE: [PATCH v5 1/3] x86/hyper-v: Suspend/resume the hypercall page for hibernation

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Dexuan Cui <decui@microsoft.com> writes:

>> From: Vitaly Kuznetsov <vkuznets@redhat.com>
>> Sent: Thursday, September 26, 2019 3:44 AM
>> > [...]
>> > +static int hv_suspend(void)
>> > +{
>> > +	union hv_x64_msr_hypercall_contents hypercall_msr;
>> > +
>> > +	/* Reset the hypercall page */
>> > +	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>> > +	hypercall_msr.enable = 0;
>> > +	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>> > +
>> 
>> (trying to think out loud, not sure there's a real issue):
>> 
>> When PV IPIs (or PV TLB flush) are enabled we do the following checks:
>> 
>> 	if (!hv_hypercall_pg)
>> 		return false;
>> 
>> or
>>         if (!hv_hypercall_pg)
>>                 goto do_native;
>> 
>> which will pass as we're not invalidating the pointer. Can we actually
>> be sure that the kernel will never try to send an IPI/do TLB flush
>> before we resume?
>> 
>> Vitaly
>
> When hv_suspend() and hv_resume() are called by syscore_suspend()
> and syscore_resume(), respectively, all the non-boot CPUs are disabled and
> only CPU0 is active and interrupts are disabled, e.g. see
>
> hibernate() -> 
>   hibernation_snapshot() ->
>     create_image() ->
>       suspend_disable_secondary_cpus()
>       local_irq_disable()
>
>       syscore_suspend()
>       swsusp_arch_suspend
>       syscore_resume
>
>       local_irq_enable
>       enable_nonboot_cpus
>
>
> So, I'm pretty sure no IPI can happen between hv_suspend() and hv_resume().
> self-IPI is not supposed to happen either, since interrupts are disabled.
>
> IMO TLB flush should not be an issue either, unless the kernel changes page
> tables between hv_suspend() and hv_resume(), which is not the case as I
> checked the related code, but it looks in theory that might happen, say, in
> the future, so if you insist we should save the variable "hv_hypercall_pg"
> to a temporary variable and set the "hv_hypercall_pg" to NULL before we
> disable the hypercall page

Let's do it as a future proof so we can keep relying on !hv_hypercall_pg
everywhere we need. No need to change this patch IMO, a follow-up would
do.

-- 
Vitaly