Re: [PATCH v4 02/10] x86/vmx: add IPT cpu feature

From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: "Kevin Tian" <kevin.tian@intel.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	tamas.lengyel@intel.com, "Wei Liu" <wl@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Michał Leszczyński" <michal.leszczynski@cert.pl>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Jun Nakajima" <jun.nakajima@intel.com>,
	xen-devel@lists.xenproject.org, luwei.kang@intel.com,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH v4 02/10] x86/vmx: add IPT cpu feature
Date: Thu, 2 Jul 2020 16:17:06 +0200	[thread overview]
Message-ID: <7ac383c2-0264-cc75-a85b-13c1fdfb0bd6@suse.com> (raw)
In-Reply-To: <7f915146-6566-e5a7-14d2-cb2319838562@xen.org>

On 02.07.2020 16:14, Julien Grall wrote:
> Hi,
> 
> On 02/07/2020 14:30, Jan Beulich wrote:
>> On 02.07.2020 11:57, Julien Grall wrote:
>>> Hi,
>>>
>>> On 02/07/2020 10:18, Jan Beulich wrote:
>>>> On 02.07.2020 10:54, Julien Grall wrote:
>>>>>
>>>>>
>>>>> On 02/07/2020 09:50, Jan Beulich wrote:
>>>>>> On 02.07.2020 10:42, Julien Grall wrote:
>>>>>>> On 02/07/2020 09:29, Jan Beulich wrote:
>>>>>>>> I'm with Andrew here, fwiw, as long as the little bit of code that
>>>>>>>> is actually put in common/ or include/xen/ doesn't imply arbitrary
>>>>>>>> restrictions on acceptable values.
>>>>>>> Well yes the code is simple. However, the code as it is wouldn't be
>>>>>>> usuable on other architecture without additional work (aside arch
>>>>>>> specific code). For instance, there is no way to map the buffer outside
>>>>>>> of Xen as it is all x86 specific.
>>>>>>>
>>>>>>> If you want the allocation to be in the common code, then the
>>>>>>> infrastructure to map/unmap the buffer should also be in common code.
>>>>>>> Otherwise, there is no point to allocate it in common.
>>>>>>
>>>>>> I don't think I agree here - I see nothing wrong with exposing of
>>>>>> the memory being arch specific, when allocation is generic. This
>>>>>> is no different from, in just x86, allocation logic being common
>>>>>> to PV and HVM, but exposing being different for both.
>>>>>
>>>>> Are you suggesting that the way it would be exposed may be different for
>>>>> other architecture?
>>>>
>>>> Why not? To take a possibly extreme example - consider an arch
>>>> where (for bare metal) the buffer is specified to appear at a
>>>> fixed range of addresses.
>>>
>>> I am probably missing something here... The current goal is the buffer
>>> will be mapped in the dom0. Most likely the way to map it will be using
>>> the acquire hypercall (unless you invent a brand new one...).
>>>
>>> For a guest, you could possibly reserve a fixed range and then map it
>>> when creating the vCPU in Xen. But then, you will likely want a fixed
>>> size... So why would you bother to ask the user to define the size?
>>
>> Because there may be the option to only populate part of the fixed
>> range?
> 
> It was yet another extreme case ;).

Yes, sure - just to demonstrate my point.

>>> Another way to do it, would be the toolstack to do the mapping. At which
>>> point, you still need an hypercall to do the mapping (probably the
>>> hypercall acquire).
>>
>> There may not be any mapping to do in such a contrived, fixed-range
>> environment. This scenario was specifically to demonstrate that the
>> way the mapping gets done may be arch-specific (here: a no-op)
>> despite the allocation not being so.
> You are arguing on extreme cases which I don't think is really helpful 
> here. Yes if you want to map at a fixed address in a guest you may not 
> need the acquire hypercall. But in most of the other cases (see has for 
> the tools) you will need it.
> 
> So what's the problem with requesting to have the acquire hypercall 
> implemented in common code?

Didn't we start out by you asking that there be as little common code
as possible for the time being? I have no issue with putting the
acquire implementation there ...

Jan