Re: [PATCH] vm_event: Implement ARM SMC events

From: Julien Grall <julien.grall@arm.com>
To: Corneliu ZUZU <czuzu@bitdefender.com>,
	Tamas K Lengyel <tamas.k.lengyel@gmail.com>
Cc: Wei Liu <wei.liu2@citrix.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Keir Fraser <keir@xen.org>,
	Razvan Cojocaru <rcojocaru@bitdefender.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH] vm_event: Implement ARM SMC events
Date: Wed, 13 Apr 2016 11:52:22 +0100	[thread overview]
Message-ID: <570E24E6.8030207@arm.com> (raw)
In-Reply-To: <570E0967.3070600@bitdefender.com>

Hello Corneliu,

On 13/04/16 09:55, Corneliu ZUZU wrote:
> On 4/12/2016 8:24 PM, Tamas K Lengyel wrote:
> Another issue came to my mind: "HVC #imm", if handled through the
> hvm-ops code, currently requires setting other registers to predefined
> values before the HVC is actually issued. That would imply additional
> effort to save/restore those registers if an external privileged domain
> would want to set guest breakpoints. Given that, if we were to use HVC
> for sw-bkpts, IMO it would be nice if the hvm-ops code architecture
> would be slightly changed such that -lone- "HVM #imm" calls would be
> achievable for some use cases, such as this.

That is not true. All the hypercalls are using the same #imm 
(XEN_HYPERCALL_TAG = 0xEA1), which indeed requires specific value in 
various registers to differentiate the HVM-ops.

It's up to us to define the requirements for the other #imm. For 
instance there are some #imm allocated for debugging (see do_debug_trap) 
that will dump the content of the registers.

>
>>
>> So what I will do instead of issuing a software_breakpoint vm_event
>> for SMCs, I'll introduce a new type, say
>> VM_EVENT_REASON_PRIVILEGED_CALL, that can be used to forward both
>> hypercalls and SMCs to a monitoring guest. This would also allow us to
>> use the software_breakpoint type for the actual software breakpoint
>> events in the future.
>>
>> Tamas
>
> Isn't the HVC-part already achieved by guest-request vm-events? Maybe
> tying this vm-event specifically to SMC (in which case the name could be
> something like VM_EVENT_REASON_SECURE_CALL) and thus making it
> ARM-specific would avoid that redundancy?

I would create 2 distinct events for HVC and SMC. It would make easier 
to differentiate them in userspace.

Regards,

-- 
Julien Grall

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