Re: [Xen-devel] [PATCH v9 3/6] sysctl / libxl: report whether IOMMU/HAP page table sharing is supported

From: Jan Beulich <jbeulich@suse.com>
To: Paul Durrant <Paul.Durrant@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	DavidScott <dave@recoil.org>, "Tim \(Xen.org\)" <tim@xen.org>,
	George Dunlap <George.Dunlap@citrix.com>,
	JulienGrall <julien.grall@arm.com>,
	Christian Lindig <christian.lindig@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Anthony Perard <anthony.perard@citrix.com>,
	IanJackson <Ian.Jackson@citrix.com>
Subject: Re: [Xen-devel] [PATCH v9 3/6] sysctl / libxl: report whether IOMMU/HAP page table sharing is supported
Date: Thu, 12 Sep 2019 15:28:09 +0200	[thread overview]
Message-ID: <9db1e764-e19e-a3fc-7d51-515d5625682f@suse.com> (raw)
In-Reply-To: <db42dfe903484b948cd82ad5d2a46844@AMSPEX02CL03.citrite.net>

On 12.09.2019 15:18, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: 12 September 2019 14:04
>> To: Paul Durrant <Paul.Durrant@citrix.com>
>> Cc: xen-devel@lists.xenproject.org; Julien Grall <julien.grall@arm.com>; Andrew Cooper
>> <Andrew.Cooper3@citrix.com>; Anthony Perard <anthony.perard@citrix.com>; Christian Lindig
>> <christian.lindig@citrix.com>; George Dunlap <George.Dunlap@citrix.com>; Ian Jackson
>> <Ian.Jackson@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>; Konrad Rzeszutek Wilk
>> <konrad.wilk@oracle.com>; DavidScott <dave@recoil.org>; Tim (Xen.org) <tim@xen.org>; Wei Liu
>> <wl@xen.org>
>> Subject: Re: [PATCH v9 3/6] sysctl / libxl: report whether IOMMU/HAP page table sharing is supported
>>
>> On 12.09.2019 13:17, Paul Durrant wrote:
>>> --- a/xen/arch/arm/sysctl.c
>>> +++ b/xen/arch/arm/sysctl.c
>>> @@ -15,6 +15,9 @@
>>>  void arch_do_physinfo(struct xen_sysctl_physinfo *pi)
>>>  {
>>>      pi->capabilities |= XEN_SYSCTL_PHYSCAP_hvm | XEN_SYSCTL_PHYSCAP_hap;
>>> +
>>> +    if ( iommu_enabled && iommu_hap_pt_share )
>>> +        pi->capabilities |= XEN_SYSCTL_PHYSCAP_iommu_hap_pt_share;
>>>  }
>>
>> I think this should be done in common code.
>>
>>> --- a/xen/arch/x86/sysctl.c
>>> +++ b/xen/arch/x86/sysctl.c
>>> @@ -164,7 +164,12 @@ void arch_do_physinfo(struct xen_sysctl_physinfo *pi)
>>>      if ( IS_ENABLED(CONFIG_PV) )
>>>          pi->capabilities |= XEN_SYSCTL_PHYSCAP_pv;
>>>      if ( hvm_hap_supported() )
>>> +    {
>>>          pi->capabilities |= XEN_SYSCTL_PHYSCAP_hap;
>>> +
>>> +        if ( iommu_enabled && iommu_hap_pt_share )
>>> +            pi->capabilities |= XEN_SYSCTL_PHYSCAP_iommu_hap_pt_share;
>>> +    }
>>>  }
>>
>> And if it's important to not have the bit set when !hvm_hap_supported(),
> 
> ...and that's what it’s not in common code... there is no hvm_hap_supported() for Arm
> 
>> then iommu_hap_pt_share should be cleared in __init code in this case.
> 
> That would have been an alternative approach if you'd not wanted
> it #defined in patch #5. (Yes it's a later patch in the series,
> but this one is later chronologically and I didn't want to
> invalidate the other patch.)
> I could perhaps implement hvm_hap_supported() for Arm?

Well, implementing it for Arm is an option (at which point
XEN_SYSCTL_PHYSCAP_hap could also move to common code). But
personally I'd advise against providing more stubs than
necessary on Arm. I may not understand your remark regarding
patch 5: There's no problem with iommu_hap_pt_share now being
a #define on Arm - whether the system is HVM capable is an
x86 specific decision anyway, and hence the clearing of the
flag could occur e.g. in hvm_enable() when it bails early.
And btw., with !CONFIG_HVM it could be a #define on x86 as
well, just to "false" then.

Jan

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