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Subject: Re: [PATCH 11/19] KVM: PPC: Book3S HV: add support for the XIVE
 native exploitation mode hcalls
To: Paul Mackerras <paulus@ozlabs.org>
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From: =?UTF-8?Q?C=c3=a9dric_Le_Goater?= <clg@kaod.org>
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Date: Wed, 23 Jan 2019 11:48:56 +0100
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Cc: kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
 David Gibson <david@gibson.dropbear.id.au>
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On 1/23/19 11:26 AM, Paul Mackerras wrote:
> On Wed, Jan 23, 2019 at 09:48:31AM +0100, Cédric Le Goater wrote:
>> On 1/23/19 7:44 AM, Benjamin Herrenschmidt wrote:
>>> On Tue, 2019-01-22 at 16:23 +1100, Paul Mackerras wrote:
>>>> Why do we need to provide real-mode versions of these hypercall
>>>> handlers?  I thought these hypercalls would only get called
>>>> infrequently, and in any case certainly much less frequently than once
>>>> per interrupt delivered.  If they are infrequent, then let's leave out
>>>> the real-mode version and just handle them in book3s_hv.c.
>>>
>>> Agreed with the exception maybe of H_INT_ESB
>>
>> ok. 
>>
>> Some of these hcalls are really simple and only getting local info from 
>> the host (h_int_get_*). I thought handling the hcall ASAP was a preferred 
>> practice, even if the hcall is not called frequently. Isn't it ?
> 
> If we are going to handle a given hcall in the kernel at all, then we
> have to have a virtual mode handler.  If we have a real-mode handler
> as well then we in general incur a certain amount of code duplication
> with consequent maintenance costs and possibility of bugs.  So we
> generally only have real-mode handlers for the hcalls where it is
> critical to minimize the latency.  From what Ben is saying that would
> only be H_INT_ESB, and maybe not even that.

ok. and yes, even the H_INT_ESB is questionable as this is really a rare
configuration. 

> If H_INT_ESB is only used for LSIs, then is a guest going to be using
> it at all?  My understanding was that with XIVE, only a small number
> of interrupts that are to do with system management functions are
> LSIs; all of the interrupts relating to PCI-e devices are MSIs.  So do
> we actually have a real high-frequency use case for LSIs in a guest?

The guest should be using a rtl8139 or a e1000 NIC under QEMU/KVM, which 
is not the common scenario. 

> For now I would prefer that you remove all the real-mode hcall
> handlers.  We can add them later if we get performance data showing
> that they are needed.

ok. I will.

> Regarding whether or not to have a given hcall handler in the kernel
> at all - if there is for example an hcall which is just called once
> on guest startup, and its function is just to provide information to
> the guest, and QEMU has that information, then why not have that hcall
> implemented by QEMU?  Are any of the hcalls like that?
> 
> For example, if H_INT_GET_SOURCE_INFO was implemented in QEMU, could
> we then remove the VC_BASE thing from the xive device?

Yes. H_INT_GET_SOURCE_INFO looks like a good candidate, all info should
be in QEMU and there are no OPAL calls, and we would get rid of the 
VC_BASE kvm device ioctl at the same time.

Thanks,

C.