[PATCH 1/14] i386 / Make write ldt return error code

[PATCH 1/14] i386 / Make write ldt return error code

[zach]

Xen requires error returns from the hypercall to update LDT entries,
and this generates completely equivalent code on native.

Patch-base: 2.6.13-rc5-mm1
Patch-keys: i386 desc ldt paravirt xen
Signed-off-by: Zachary Amsden <zach@vmware.com>

Index: linux-2.6.13/include/asm-i386/mach-default/mach_desc.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/mach-default/mach_desc.h	2005-08-09 18:19:39.000000000 -0700
+++ linux-2.6.13/include/asm-i386/mach-default/mach_desc.h	2005-08-09 18:24:10.000000000 -0700
@@ -62,11 +62,12 @@
 	_set_tssldt_desc(&per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_LDT], (int)addr, ((size << 3)-1), 0x82);
 }
 
-static inline void write_ldt_entry(void *ldt, int entry, __u32 entry_a, __u32 entry_b)
+static inline int write_ldt_entry(void *ldt, int entry, __u32 entry_a, __u32 entry_b)
 {
 	__u32 *lp = (__u32 *)((char *)ldt + entry*8);
 	*lp = entry_a;
 	*(lp+1) = entry_b;
+	return 0;
 }
 
 #if TLS_SIZE != 24
Index: linux-2.6.13/arch/i386/kernel/ldt.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/ldt.c	2005-08-09 18:19:37.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/ldt.c	2005-08-09 18:22:56.000000000 -0700
@@ -221,8 +221,7 @@
 
 	/* Install the new entry ...  */
 install:
-	write_ldt_entry(mm->context.ldt, ldt_info.entry_number, entry_1, entry_2);
-	error = 0;
+	error = write_ldt_entry(mm->context.ldt, ldt_info.entry_number, entry_1, entry_2);
 
 out_unlock:
 	up(&mm->context.sem);

Re: [PATCH 1/14] i386 / Make write ldt return error code

[Chris Wright]

* zach@vmware.com (zach@vmware.com) wrote:
> Xen requires error returns from the hypercall to update LDT entries,
> and this generates completely equivalent code on native.

The whole lot looks quite nice.  I've sucked them into a git tree 
with the full set until Andrew's back.  If it's useful we can work
against that tree.

rsync://rsync.kernel.org/pub/scm/linux/kernel/git/chrisw/virt-2.6

thanks,
-chris

Re: [PATCH 1/14] i386 / Make write ldt return error code

[Andi Kleen]

On Wed, Aug 10, 2005 at 09:52:51PM -0700, zach@vmware.com wrote:
> Xen requires error returns from the hypercall to update LDT entries,
> and this generates completely equivalent code on native.

I don't think that is something we want. Nothing in the callers will check
the errors anyways. If Xen has such a requirement it should kill
the guest when it is violated, otherwise they will be ignored.

I would drop that one.

-Andi

Re: [PATCH 1/14] i386 / Make write ldt return error code

[Chris Wright]

* Andi Kleen (ak@suse.de) wrote:
> On Wed, Aug 10, 2005 at 09:52:51PM -0700, zach@vmware.com wrote:
> > Xen requires error returns from the hypercall to update LDT entries,
> > and this generates completely equivalent code on native.
> 
> I don't think that is something we want. Nothing in the callers will check
> the errors anyways. If Xen has such a requirement it should kill
> the guest when it is violated, otherwise they will be ignored.

In this case the callers do propagate the error (unless you mean
userspace doesn't check return value from syscall, which is same problem
if copy_from_user failed, for example).  Xen has done some more wrapping
of the hypercalls to be effectively synchronous exceptions on error.
Keir, Ian, is that preferred method going forward?

thanks,
-chris

Re: [PATCH 1/14] i386 / Make write ldt return error code

[Andi Kleen]

On Tue, Aug 16, 2005 at 05:06:18PM -0700, Chris Wright wrote:
> * Andi Kleen (ak@suse.de) wrote:
> > On Wed, Aug 10, 2005 at 09:52:51PM -0700, zach@vmware.com wrote:
> > > Xen requires error returns from the hypercall to update LDT entries,
> > > and this generates completely equivalent code on native.
> > 
> > I don't think that is something we want. Nothing in the callers will check
> > the errors anyways. If Xen has such a requirement it should kill
> > the guest when it is violated, otherwise they will be ignored.
> 
> In this case the callers do propagate the error (unless you mean
> userspace doesn't check return value from syscall, which is same problem
> if copy_from_user failed, for example).  Xen has done some more wrapping

Nothing checks them in user space.

Also how would you handle them anyways? It just doesn't make sense.

-Andi

Re: [PATCH 1/14] i386 / Make write ldt return error code

[Chris Wright]

* Andi Kleen (ak@suse.de) wrote:
> On Tue, Aug 16, 2005 at 05:06:18PM -0700, Chris Wright wrote:
> > In this case the callers do propagate the error (unless you mean
> > userspace doesn't check return value from syscall, which is same problem
> > if copy_from_user failed, for example).  Xen has done some more wrapping
> 
> Nothing checks them in user space.
> 
> Also how would you handle them anyways? It just doesn't make sense.

Yes, I see your point, although copy_from_user failing has similar issue
if userspace isn't checking errors.  But this one is not critical.

thanks,
-chris