[Xen-devel] [PATCH] tools/libxc: Construct 32bit PV guests with L3 A/D bits set

Andrew Cooper posted 1 patch 36 weeks ago
Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/xen tags/patchew/20200114123921.30850-1-andrew.cooper3@citrix.com
tools/libxc/xc_dom_x86.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

[Xen-devel] [PATCH] tools/libxc: Construct 32bit PV guests with L3 A/D bits set

Posted by Andrew Cooper 36 weeks ago
With the 32 PAE build of Xen gone, 32bit PV guests' top level pagetables no
longer behave exactly like PAE in hardware.

They should have A/D bits set, for the same performance reasons as apply to
other levels.  This brings the domain builder in line with how Xen constructs
a 32bit dom0.

As a purely code improvement, make use of range notation to initialise
identical values in adjacent array elements.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Ian Jackson <Ian.Jackson@citrix.com>

It turns out that neither Xen nor libxc sets dirty bits on L1 pagetables, but
I can't spot any documented reason for this.  My best guess is to avoid having
D/RO mappings of pagetables, but this isn't going to trip up a PV guest to
begin with.  Thoughts on reducing the performance cost there as well?
---
 tools/libxc/xc_dom_x86.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/tools/libxc/xc_dom_x86.c b/tools/libxc/xc_dom_x86.c
index 1897decedb..425b129146 100644
--- a/tools/libxc/xc_dom_x86.c
+++ b/tools/libxc/xc_dom_x86.c
@@ -277,8 +277,8 @@ static int alloc_pgtables_x86_32_pae(struct xc_dom_image *dom)
         .levels = PGTBL_LEVELS_I386,
         .vaddr_mask = bits_to_mask(VIRT_BITS_I386),
         .lvl_prot[0] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED,
-        .lvl_prot[1] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
-        .lvl_prot[2] = _PAGE_PRESENT,
+        .lvl_prot[1 ... 2] =
+            _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
     };
     struct xc_dom_image_x86 *domx86 = dom->arch_private;
 
@@ -488,9 +488,8 @@ static int alloc_pgtables_x86_64(struct xc_dom_image *dom)
         .levels = PGTBL_LEVELS_X86_64,
         .vaddr_mask = bits_to_mask(VIRT_BITS_X86_64),
         .lvl_prot[0] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED,
-        .lvl_prot[1] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
-        .lvl_prot[2] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
-        .lvl_prot[3] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
+        .lvl_prot[1 ... 3] =
+            _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
     };
     struct xc_dom_image_x86 *domx86 = dom->arch_private;
 
-- 
2.11.0


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Re: [Xen-devel] [PATCH] tools/libxc: Construct 32bit PV guests with L3 A/D bits set

Posted by Wei Liu 36 weeks ago
On Tue, Jan 14, 2020 at 12:39:21PM +0000, Andrew Cooper wrote:
> With the 32 PAE build of Xen gone, 32bit PV guests' top level pagetables no
> longer behave exactly like PAE in hardware.
> 
> They should have A/D bits set, for the same performance reasons as apply to
> other levels.  This brings the domain builder in line with how Xen constructs
> a 32bit dom0.
> 
> As a purely code improvement, make use of range notation to initialise
> identical values in adjacent array elements.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Acked-by: Wei Liu <wl@xen.org>

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Re: [Xen-devel] [PATCH] tools/libxc: Construct 32bit PV guests with L3 A/D bits set

Posted by Jan Beulich 36 weeks ago
On 14.01.2020 13:39, Andrew Cooper wrote:
> With the 32 PAE build of Xen gone, 32bit PV guests' top level pagetables no
> longer behave exactly like PAE in hardware.
> 
> They should have A/D bits set, for the same performance reasons as apply to
> other levels.  This brings the domain builder in line with how Xen constructs
> a 32bit dom0.
> 
> As a purely code improvement, make use of range notation to initialise
> identical values in adjacent array elements.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>

> It turns out that neither Xen nor libxc sets dirty bits on L1 pagetables, but
> I can't spot any documented reason for this.  My best guess is to avoid having
> D/RO mappings of pagetables, but this isn't going to trip up a PV guest to
> begin with.  Thoughts on reducing the performance cost there as well?

I'd ack such a patch.

> --- a/tools/libxc/xc_dom_x86.c
> +++ b/tools/libxc/xc_dom_x86.c
> @@ -277,8 +277,8 @@ static int alloc_pgtables_x86_32_pae(struct xc_dom_image *dom)
>          .levels = PGTBL_LEVELS_I386,
>          .vaddr_mask = bits_to_mask(VIRT_BITS_I386),
>          .lvl_prot[0] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED,
> -        .lvl_prot[1] = _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,
> -        .lvl_prot[2] = _PAGE_PRESENT,
> +        .lvl_prot[1 ... 2] =
> +            _PAGE_PRESENT|_PAGE_RW|_PAGE_ACCESSED|_PAGE_DIRTY|_PAGE_USER,

Perhaps worthwhile leaving a comment as to the difference to what
readers might expect for PAE L3 entries?

Jan

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