From: Julien Grall <julien.grall@arm.com>
To: Tamas K Lengyel <tamas.lengyel@zentific.com>,
Jan Beulich <JBeulich@suse.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [PATCH v8 4/6] arm/vm_event: get/set registers
Date: Wed, 6 Jul 2016 22:12:06 +0100 [thread overview]
Message-ID: <abfe2c77-87fb-0ca3-8efd-e924bcb8a74e@arm.com> (raw)
In-Reply-To: <CAErYnsh4ZXXZBFuyk49407j63Okh9BrxX_fWNet8bDeQrPbciw@mail.gmail.com>
On 06/07/2016 20:23, Tamas K Lengyel wrote:
> On Wed, Jul 6, 2016 at 1:43 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 05.07.16 at 20:37, <tamas.lengyel@zentific.com> wrote:
>>> +struct vm_event_regs_arm64 {
>>> + uint64_t x0;
>>> + uint64_t x1;
>>> + uint64_t x2;
>>> + uint64_t x3;
>>> + uint64_t x4;
>>> + uint64_t x5;
>>> + uint64_t x6;
>>> + uint64_t x7;
>>> + uint64_t x8;
>>> + uint64_t x9;
>>> + uint64_t x10;
>>> + uint64_t x11;
>>> + uint64_t x12;
>>> + uint64_t x13;
>>> + uint64_t x14;
>>> + uint64_t x15;
>>> + uint64_t x16;
>>> + uint64_t x17;
>>> + uint64_t x18;
>>> + uint64_t x19;
>>> + uint64_t x20;
>>> + uint64_t x21;
>>> + uint64_t x22;
>>> + uint64_t x23;
>>> + uint64_t x24;
>>> + uint64_t x25;
>>> + uint64_t x26;
>>> + uint64_t x27;
>>> + uint64_t x28;
>>> + uint64_t x29;
>>> + uint64_t x30;
>>> + uint64_t pc;
>>> +};
>>
>> Isn't the stack pointer a fully separate register in aarch64? Not
>> making available something as essential as that seems wrong to
>> me.
>>
>
> The register is available for access already, so unless there is an
> actual use-case that requires it to be transmitted through vm_event I
> don't see the point for transmitting it. So as I mentioned in my other
> response, I'm inclined to reduce this patch to the bare essentials my
> use-case requires at this point and leave the extensions up for the
> future when - and if - it will be of use. Since this patch is just an
> optimization, if transmitting such reduced set is not acceptable for
> some reason, I'll forgo this patch entirely.
Here we go with again the same argument: "this is not necessary for my
use-case". This data structure is part of the ABI between the hypervisor
and the vm-event app, i.e modifying this structure for adding ARM64/ARM
registers will result to an incompatibility with a previous version of
the hypervisor. Better to do it now than in a couple of years when
vm-event will have more users. I agree that it is time consuming to get
an ABI correct, but it will save users to recompile/ship another version
of their vm-event app because of this incompatibility.
As mentioned in a previous thread, the main use-case for trapping an SMC
is emulating the call, hence a vm-event app would want to have access to
the general-purpose registers. And yes, I know that your use-case is
different and does not require those registers, I already expressed my
concerns about it.
Now, if you drop this patch, how will you retrieve the vCPU register? I
guess via DOMCTL_getvcpucontext? If so, if the vCPU has not been paused,
you will get a context which is different compare to the time the
vm-event has occurred. And yes, I know that in your use-case the vCPU is
paused. This call will always be more expensive than passing the
registers with event.
Anyway, I really don't think we ask for something really difficult to
accomplish.
Regards,
--
Julien Grall
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next prev parent reply other threads:[~2016-07-06 21:12 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-05 18:37 [PATCH v8 1/6] monitor: rename and relocate vm_event_monitor_traps Tamas K Lengyel
2016-07-05 18:37 ` [PATCH v8 2/6] arm: filter SMC exceptions with failed condition checks Tamas K Lengyel
2016-07-06 17:31 ` Julien Grall
2016-07-06 18:52 ` Tamas K Lengyel
2016-07-05 18:37 ` [PATCH v8 3/6] monitor: ARM SMC events Tamas K Lengyel
2016-07-05 18:37 ` [PATCH v8 4/6] arm/vm_event: get/set registers Tamas K Lengyel
2016-07-06 7:43 ` Jan Beulich
2016-07-06 7:59 ` Razvan Cojocaru
2016-07-06 17:39 ` Julien Grall
2016-07-06 19:23 ` Tamas K Lengyel
2016-07-06 21:12 ` Julien Grall [this message]
2016-07-06 22:01 ` Tamas K Lengyel
2016-07-06 18:04 ` Julien Grall
2016-07-06 19:12 ` Tamas K Lengyel
2016-07-07 8:23 ` Jan Beulich
2016-07-07 9:46 ` Julien Grall
2016-07-07 9:57 ` Jan Beulich
2016-07-07 10:09 ` Julien Grall
2016-07-07 15:53 ` Tamas K Lengyel
2016-07-05 18:37 ` [PATCH v8 5/6] tools/libxc: add xc_monitor_privileged_call Tamas K Lengyel
2016-07-05 18:37 ` [PATCH v8 6/6] tools/xen-access: add test-case for ARM SMC Tamas K Lengyel
2016-07-07 10:05 ` Julien Grall
2016-07-07 15:54 ` Tamas K Lengyel
2016-07-05 19:15 ` [PATCH v8 1/6] monitor: rename and relocate vm_event_monitor_traps Razvan Cojocaru
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