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Subject: Re: [PATCH 3/6] x86: Add nopv parameter to disable PV extensions
To:     Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org
Cc:     tglx@linutronix.de, mingo@kernel.org, bp@alien8.de, hpa@zytor.com,
        boris.ostrovsky@oracle.com, sstabellini@kernel.org,
        xen-devel@lists.xenproject.org
References: <1561294903-6166-1-git-send-email-zhenzhong.duan@oracle.com>
 <1561294903-6166-3-git-send-email-zhenzhong.duan@oracle.com>
 <9e60cea2-a15f-b816-9049-f22be14c04b2@suse.com>
From:   Zhenzhong Duan <zhenzhong.duan@oracle.com>
Organization: Oracle Corporation
Message-ID: <cf315d9a-7027-aa8c-1cac-9be9b734811c@oracle.com>
Date:   Mon, 24 Jun 2019 21:50:16 +0800
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On 2019/6/24 21:18, Juergen Gross wrote:
> On 23.06.19 15:01, Zhenzhong Duan wrote:
>> In virtualization environment, PV extensions (drivers, interrupts,
>> timers, etc) are enabled in the majority of use cases which is the
>> best option.
>>
>> However, in some cases (kexec not fully working, benchmarking)
>> we want to disable PV extensions. As such introduce the
>> 'nopv' parameter that will do it.
>>
>> There is already 'xen_nopv' parameter for XEN platform but not for
>> others. 'xen_nopv' can then be removed with this change.
>>
>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
>> Cc: xen-devel@lists.xenproject.org
>> ---
>>   Documentation/admin-guide/kernel-parameters.txt |  4 ++++
>>   arch/x86/kernel/cpu/hypervisor.c                | 11 +++++++++++
>>   2 files changed, 15 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt 
>> b/Documentation/admin-guide/kernel-parameters.txt
>> index 138f666..b352f36 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -5268,6 +5268,10 @@
>>               improve timer resolution at the expense of processing
>>               more timer interrupts.
>>   +    nopv=        [X86]
>> +            Disables the PV optimizations forcing the guest to run
>> +            as generic guest with no PV drivers.
>> +
>>       xirc2ps_cs=    [NET,PCMCIA]
>>               Format:
>> <irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
>> diff --git a/arch/x86/kernel/cpu/hypervisor.c 
>> b/arch/x86/kernel/cpu/hypervisor.c
>> index 479ca47..4f2c875 100644
>> --- a/arch/x86/kernel/cpu/hypervisor.c
>> +++ b/arch/x86/kernel/cpu/hypervisor.c
>> @@ -85,10 +85,21 @@ static void __init copy_array(const void *src, 
>> void *target, unsigned int size)
>>               to[i] = from[i];
>>   }
>>   +static bool nopv;
>> +static __init int xen_parse_nopv(char *arg)
>> +{
>> +    nopv = true;
>> +    return 0;
>> +}
>> +early_param("nopv", xen_parse_nopv);
>> +
>>   void __init init_hypervisor_platform(void)
>>   {
>>       const struct hypervisor_x86 *h;
>>   +    if (nopv)
>> +        return;
>> +
>
> Oh, this is no good idea.
>
> There are guest types which just won't work without pv interfaces, like
> Xen PV and Xen PVH. Letting them fail due to just a wrong command line
> parameter is not nice, especially as the failure might be very hard to
> track down to the issue for the user.
Yes, thanks for catching.
>
> I guess you could add a "ignore_nopv" member to struct hypervisor_x86
> set to true for the mentioned guest types and call the detect functions
> only if nopv is false or ignore_nopv is true.

I think your suggestion is good, I'll rework it based on your suggestion.

Thanks

Zhenzhong