[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH 5/6] iommu: tidy up iommu_us_hap_pt() and need_iommu_pt_sync() macros


On 14/08/2019 11:27, Paul Durrant wrote:
-----Original Message-----
From: Julien Grall <julien.grall@xxxxxxx>
Sent: 14 August 2019 11:21
To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Jan Beulich' <jbeulich@xxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; 
Roger Pau Monne
<roger.pau@xxxxxxxxxx>; Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; George 
<George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Stefano 
<sstabellini@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Tim 
(Xen.org) <tim@xxxxxxx>;
Wei Liu <wl@xxxxxxx>
Subject: Re: [PATCH 5/6] iommu: tidy up iommu_us_hap_pt() and 
need_iommu_pt_sync() macros

Hi Paul,

On 14/08/2019 11:13, Paul Durrant wrote:
--- a/xen/include/xen/iommu.h
+++ b/xen/include/xen/iommu.h
@@ -268,6 +268,13 @@ struct domain_iommu {
    #define iommu_set_feature(d, f)   set_bit(f, dom_iommu(d)->features)
    #define iommu_clear_feature(d, f) clear_bit(f, dom_iommu(d)->features)

+/* Are we using the domain P2M table as its IOMMU pagetable? */
+#define iommu_use_hap_pt(d) \
+    (hap_enabled(d) && is_iommu_enabled(d) && iommu_hap_pt_share)

Does this build for Arm, seeing that there's no hap_enabled()
definition there? Or have I missed its addition earlier in this

It moved to common code sched.h in an earlier patch.

I went through the series and didn't find where hap_enabled() is defined for Arm
in this series. Do you mind pointing the exact patch?

Sorry, I wasn't clear... The change is in my other series, "use stashed domain 
create flags", which is a pre-requisite for this series (as called out in the cover 
letter). The change is made in patch #2 of that series: 

Oh. I understand this adds benefits as the implementation is now common. But the downside is hap_enabled() will now require evaluation on Arm even it is evaluates to true... This will prevent the compiler to remove any non-HAP code paths (assuming there are any in the common code).

Furthermore, 2 parts of the iommu_use_hap_pt() condition will always returning always true. But as they are non-constant, so they will always be evaluated.

It is also probably going to confuse developer as they may think non-HAP is supported on Arm. You can't find easily that both hap_enabled(...) and iommu_hap_pt_share will always evaluate to true.

So aside the common implementation, what is the real gain for Arm?


Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.