[PATCH v3 2/2] x86/dmar: zap DMAR signature for dom0 once in TBOOT case

[PATCH v3 2/2] x86/dmar: zap DMAR signature for dom0 once in TBOOT case

[Zhenzhong Duan]

When TBOOT enabled, acpi_parse_dmar() called acpi_dmar_zap() to zap
DMAR table, then tboot_parse_dmar_table() called acpi_dmar_zap() to
zap it again. This is unnecessory, remove the second call.

Some stale comments not right for current code are also removed.
No functional change.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
---
v3: fixup patch and description.

 xen/arch/x86/tboot.c |    6 ------
 1 files changed, 0 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/tboot.c b/xen/arch/x86/tboot.c
index d5a5292..1006f95 100644
--- a/xen/arch/x86/tboot.c
+++ b/xen/arch/x86/tboot.c
@@ -461,8 +461,6 @@ int __init tboot_parse_dmar_table(acpi_table_handler dmar_handler)
     if ( txt_heap_base == 0 )
         return 1;
 
-    /* map TXT heap into Xen addr space */
-
     /* walk heap to SinitMleData */
     pa = txt_heap_base;
     /* skip BiosData */
@@ -490,10 +488,6 @@ int __init tboot_parse_dmar_table(acpi_table_handler dmar_handler)
     rc = dmar_handler(dmar_table);
     xfree(dmar_table);
 
-    /* acpi_parse_dmar() zaps APCI DMAR signature in TXT heap table */
-    /* but dom0 will read real table, so must zap it there too */
-    acpi_dmar_zap();
-
     return rc;
 }
 
-- 
1.7.3

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Re: [PATCH v3 2/2] x86/dmar: zap DMAR signature for dom0 once in TBOOT case

[Jan Beulich]

>>> On 21.08.18 at 11:11, <zhenzhong.duan@oracle.com> wrote:
> @@ -490,10 +488,6 @@ int __init tboot_parse_dmar_table(acpi_table_handler dmar_handler)
>      rc = dmar_handler(dmar_table);
>      xfree(dmar_table);
>  
> -    /* acpi_parse_dmar() zaps APCI DMAR signature in TXT heap table */
> -    /* but dom0 will read real table, so must zap it there too */
> -    acpi_dmar_zap();

Interesting - now you remove the other one. I think this is actually correct,
but understanding why could _again_ have been a lot easier with a better
description. Afaict commit 6c298ecc1f ("vtd: Reinstate ACPI DMAR on
system shutdown or S3/S4/S5") did everything for this call to be
unnecessary, except for invoking the function from acpi_parse_dmar(),
which 123c779379 ("VTd/dmar: Tweak how the DMAR table is clobbered")
added several years later.

Jan



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Re: [PATCH v3 2/2] x86/dmar: zap DMAR signature for dom0 once in TBOOT case

[Tian, Kevin]

> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: Tuesday, August 21, 2018 8:32 PM
> 
> >>> On 21.08.18 at 11:11, <zhenzhong.duan@oracle.com> wrote:
> > @@ -490,10 +488,6 @@ int __init
> tboot_parse_dmar_table(acpi_table_handler dmar_handler)
> >      rc = dmar_handler(dmar_table);
> >      xfree(dmar_table);
> >
> > -    /* acpi_parse_dmar() zaps APCI DMAR signature in TXT heap table */
> > -    /* but dom0 will read real table, so must zap it there too */
> > -    acpi_dmar_zap();
> 
> Interesting - now you remove the other one. I think this is actually correct,
> but understanding why could _again_ have been a lot easier with a better
> description. Afaict commit 6c298ecc1f ("vtd: Reinstate ACPI DMAR on
> system shutdown or S3/S4/S5") did everything for this call to be
> unnecessary, except for invoking the function from acpi_parse_dmar(),
> which 123c779379 ("VTd/dmar: Tweak how the DMAR table is clobbered")
> added several years later.
> 

yes, a better description is expected here.

Thanks
Kevin

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