linux-kernel - Re: [PATCH v4 09/15] Drivers: hv: Introduce hv_output_arg_exists in hv

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Date:   Wed, 4 Oct 2023 11:27:56 -0700
From:   Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To:     Alex Ionescu <aionescu@...il.com>,
        Dexuan Cui <decui@...rosoft.com>, longli@...rosoft.com,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Cc:     linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arch@...r.kernel.org, patches@...ts.linux.dev,
        mikelley@...rosoft.com, kys@...rosoft.com, wei.liu@...nel.org,
        gregkh@...uxfoundation.org, haiyangz@...rosoft.com,
        decui@...rosoft.com, apais@...ux.microsoft.com,
        Tianyu.Lan@...rosoft.com, ssengar@...ux.microsoft.com,
        mukeshrathor@...rosoft.com, stanislav.kinsburskiy@...il.com,
        jinankjain@...ux.microsoft.com, vkuznets@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, will@...nel.org,
        catalin.marinas@....com
Subject: Re: [PATCH v4 09/15] Drivers: hv: Introduce hv_output_arg_exists in
 hv_common.c

On 10/2/2023 12:29 PM, Alex Ionescu wrote:
> Hi Nuno,
> 
> Is it possible to simply change to always allocating the output page?
> For example, the output page could be needed in scenarios where Linux
> is not running as the root partition, since certain hypercalls that a
> guest can make will still require one (I realize that's not the case
> _today_, but I don't believe this optimization buys much).

I agree - it would indeed simplify the code, and guests will probably
make use of it sooner or later.

Happy to make that change if Hyper-V guest maintainers agree.
Long, Dexuan, Michael, what do you think?

Thanks,
Nuno

> Best regards,
> Alex Ionescu
> 
> 
> On Fri, Sep 29, 2023 at 2:02 PM Nuno Das Neves
> <nunodasneves@...ux.microsoft.com> wrote:
>>
>> This is a more flexible approach for determining whether to allocate the
>> output page.
>>
>> Signed-off-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
>> Acked-by: Wei Liu <wei.liu@...nel.org>
>> ---
>>   drivers/hv/hv_common.c | 21 +++++++++++++++++----
>>   1 file changed, 17 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>> index 39077841d518..3f6f23e4c579 100644
>> --- a/drivers/hv/hv_common.c
>> +++ b/drivers/hv/hv_common.c
>> @@ -58,6 +58,14 @@ EXPORT_SYMBOL_GPL(hyperv_pcpu_input_arg);
>>   void * __percpu *hyperv_pcpu_output_arg;
>>   EXPORT_SYMBOL_GPL(hyperv_pcpu_output_arg);
>>
>> +/*
>> + * Determine whether output arg is needed
>> + */
>> +static inline bool hv_output_arg_exists(void)
>> +{
>> +       return hv_root_partition ? true : false;
>> +}
>> +
>>   static void hv_kmsg_dump_unregister(void);
>>
>>   static struct ctl_table_header *hv_ctl_table_hdr;
>> @@ -342,10 +350,12 @@ int __init hv_common_init(void)
>>          hyperv_pcpu_input_arg = alloc_percpu(void  *);
>>          BUG_ON(!hyperv_pcpu_input_arg);
>>
>> -       /* Allocate the per-CPU state for output arg for root */
>> -       if (hv_root_partition) {
>> +       if (hv_output_arg_exists()) {
>>                  hyperv_pcpu_output_arg = alloc_percpu(void *);
>>                  BUG_ON(!hyperv_pcpu_output_arg);
>> +       }
>> +
>> +       if (hv_root_partition) {
>>                  hv_synic_eventring_tail = alloc_percpu(u8 *);
>>                  BUG_ON(hv_synic_eventring_tail == NULL);
>>          }
>> @@ -375,7 +385,7 @@ int hv_common_cpu_init(unsigned int cpu)
>>          u8 **synic_eventring_tail;
>>          u64 msr_vp_index;
>>          gfp_t flags;
>> -       int pgcount = hv_root_partition ? 2 : 1;
>> +       int pgcount = hv_output_arg_exists() ? 2 : 1;
>>          void *mem;
>>          int ret;
>>
>> @@ -393,9 +403,12 @@ int hv_common_cpu_init(unsigned int cpu)
>>                  if (!mem)
>>                          return -ENOMEM;
>>
>> -               if (hv_root_partition) {
>> +               if (hv_output_arg_exists()) {
>>                          outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
>>                          *outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
>> +               }
>> +
>> +               if (hv_root_partition) {
>>                          synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
>>                          *synic_eventring_tail = kcalloc(HV_SYNIC_SINT_COUNT, sizeof(u8),
>>                                                          flags);
>> --
>> 2.25.1
>>
>>