Re: [PATCH RFC 1/4] x86/mm: Shadow and p2m changes for PV mem_access

From: "Aravindh Puthiyaparambil (aravindp)" <aravindp@cisco.com>
To: Tamas Lengyel <tamas.lengyel@zentific.com>
Cc: Keir Fraser <keir@xen.org>,
	Ian Campbell <ian.campbell@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Jan Beulich <jbeulich@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH RFC 1/4] x86/mm: Shadow and p2m changes for PV mem_access
Date: Tue, 29 Apr 2014 23:36:25 +0000	[thread overview]
Message-ID: <97A500D504438F4ABC02EBA81613CC63317E7B1A@xmb-aln-x02.cisco.com> (raw)
In-Reply-To: <CAErYnsitFqBcFsGbDJgBYRA1ogO68-eaCW3fMeqbxJ4H78Nyvw@mail.gmail.com>

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+    /* For PV domains we only support rw, rx, rx2rw, rwx access permissions */

There is a bit of discrepancy between the comment and the code here. Also, could you briefly explain why only these permissions are supported?

I will add “r” to the comment.

Only these permissions are supported as the regular page tables unlike extended page tables (EPT) does not have an explicit R bit. It only has a present bit and hence there is no easy way to do permissions like execute only. And of course permissions like write or write-execute cannot be supported in both cases.

+    if ( unlikely(p2ma != p2m_access_r &&
+                  p2ma != p2m_access_rw &&
+                  p2ma != p2m_access_rx &&
+                  p2ma != p2m_access_rwx &&
+                  p2ma != p2m_access_rx2rw) )
+        return -EINVAL;

+    /* For PV domains we only support r, rw, rx, rwx access permissions */

Code/comment discrepancy again.

I will add “rx2rw” to the comment.

+    if ( p2m->default_access != p2m_access_r &&
+         p2m->default_access != p2m_access_rw &&
+         p2m->default_access != p2m_access_rx &&
+         p2m->default_access != p2m_access_rwx &&
+         p2m->default_access != p2m_access_rx2rw )
+        return -EINVAL;
+

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