linux-kernel - Re: [PATCH v2 2/2] x86/hyperv: remove on-stack cpumask from hv_send_ipi_mask

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Message-ID: <87k0ir63au.fsf@vitty.brq.redhat.com>
Date:   Tue, 05 Oct 2021 14:53:29 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>, Wei Liu <wei.liu@...nel.org>
Cc:     Michael Kelley <mikelley@...rosoft.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, decui@...rosoft.com,
        sthemmin@...rosoft.com, Wei Liu <wei.liu@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        Linux on Hyper-V List <linux-hyperv@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] x86/hyperv: remove on-stack cpumask from
 hv_send_ipi_mask_allbutself

Thomas Gleixner <tglx@...utronix.de> writes:

> Wei!
>

Not Wei here but I don't see the question answered on the mailing list
so let me give my thoughts.

> On Fri, Sep 10 2021 at 18:57, Wei Liu wrote:
>> -static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector)
>> +static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector,
>> +		bool exclude_self)
>>  {
>>  	struct hv_send_ipi_ex **arg;
>>  	struct hv_send_ipi_ex *ipi_arg;
>> @@ -123,7 +124,10 @@ static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector)
>>  
>>  	if (!cpumask_equal(mask, cpu_present_mask)) {
>
> Not part of that patch, but is checking cpu_present_mask correct here?
> If so then this really lacks a comment for the casual reader.

It seems it *was* correct prior to 'exclude_self': the idea is that for
everything but 'cpu_present_mask' we use HV_GENERIC_SET_SPARSE_4K
format, for 'cpu_present_mask' we just use 'all' (HV_GENERIC_SET_ALL)
to avoid specifying individual CPUs. 

>
>>  		ipi_arg->vp_set.format = HV_GENERIC_SET_SPARSE_4K;
>> -		nr_bank = cpumask_to_vpset(&(ipi_arg->vp_set), mask);
>> +		if (exclude_self)
>> +			nr_bank = cpumask_to_vpset_noself(&(ipi_arg->vp_set), mask);
>> +		else
>> +			nr_bank = cpumask_to_vpset(&(ipi_arg->vp_set), mask);
>>  	}
>
> But, what happens in the case that mask == cpu_present_mask and
> exclude_self == true?
>
> AFAICT it ends up sending the IPI to all CPUs including self:
>
> 	if (!nr_bank)
> 		ipi_arg->vp_set.format = HV_GENERIC_SET_ALL;
>
> Not entirely correct, right?

It's not, I think we need something like (completely untested) 

diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
index 32a1ad356c18..80b7660208e4 100644
--- a/arch/x86/hyperv/hv_apic.c
+++ b/arch/x86/hyperv/hv_apic.c
@@ -122,17 +122,17 @@ static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector,
        ipi_arg->reserved = 0;
        ipi_arg->vp_set.valid_bank_mask = 0;
 
-       if (!cpumask_equal(mask, cpu_present_mask)) {
+       if (!cpumask_equal(mask, cpu_present_mask) || exclude_self) {
                ipi_arg->vp_set.format = HV_GENERIC_SET_SPARSE_4K;
                if (exclude_self)
                        nr_bank = cpumask_to_vpset_noself(&(ipi_arg->vp_set), mask);
                else
                        nr_bank = cpumask_to_vpset(&(ipi_arg->vp_set), mask);
-       }
-       if (nr_bank < 0)
-               goto ipi_mask_ex_done;
-       if (!nr_bank)
+               if (nr_bank =< 0)
+                       goto ipi_mask_ex_done;
+       } else {
                ipi_arg->vp_set.format = HV_GENERIC_SET_ALL;
+       }
 
        status = hv_do_rep_hypercall(HVCALL_SEND_IPI_EX, 0, nr_bank,
                              ipi_arg, NULL);

here. Wei, I can test and send this out if you're not on it already.

-- 
Vitaly