allocate_empty_lowmem_region hypervisor function

It's only used in the backend drivers, so I guess the build is trying to omit that as dead code as far as a domU is concerned. IIRC the function is pure Linux-isms with no Xen-specifics, so it should be fine to build into a domU (or even x86 Linux).

Cheers,
Mark

Could it be added to DomU, otherwise there is no way to map foreign pages using the grant tables in unprivileged domains. I thought the point of grant tables was to allow inter domain communication between non-privileged domains? I could easily supply a patch if people think this should be possible.

It uses the balloon driver and hypervisor calls, so I don't think it would work in x86 Linux, but it works fine in DomU.

Thanks,

Ross

Yep. The existing uses of grant tables (Netif, XenFS) all used page exchange, so they would have had somewhere to map incoming pages (i.e. empty slots left by frames they'd flipped to the server). If you're running a backend then you really do need this function to make some space to map with.

The lightest-weight solution (minimising dead code) would be to have this only built if you are building a backend driver. Then again, building it unconditionally won't add much code and is a rather smaller patch.

In any case, I think we should enable backends without physdev access so a patch would be good to see.

Oops, I was thinking of a different, related call in the backend :-) But yes, allocate_empty_lowmem will work fine in domU.

Cheers,
Mark

Below is a patch to the enable allocate_empty_lowmem_region in on-privileged domains, so that backend drivers can map granted foreign pages even if they are running in an unprivileged domain. (as per the "allocate_empty_lowmem_region hypervisor function" thread discussion)

Thanks

Ross

# HG changeset patch
# User [email protected]
# Node ID b235cde45efbb903d3e0e9626df6a90adde20577 # Parent 2f743309f21afdc6adaef01421da40f7ff1e8b0d Add allocate_empty_lowmem_region() for unprivileged domains

diff -r 2f743309f21a -r b235cde45efb linux-2.6-xen-sparse/arch/xen/i386/mm/hypervisor.c
--- a/linux-2.6-xen-sparse/arch/xen/i386/mm/hypervisor.c Tue Aug 2
12:37:37 2005
+++ b/linux-2.6-xen-sparse/arch/xen/i386/mm/hypervisor.c Tue Aug 2
13:58:30 2005
@@ -35,6 +35,7 @@
#include <asm/pgtable.h>
#include <asm-xen/hypervisor.h>
#include <asm-xen/balloon.h>
+#include <linux/module.h>
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,0) #include <linux/percpu.h>
#include <asm/tlbflush.h>
@@ -312,7 +313,6 @@

     balloon_unlock(flags);

}

-#ifdef CONFIG_XEN_PHYSDEV_ACCESS

unsigned long allocate_empty_lowmem_region(unsigned long pages) {
@@ -360,4 +360,4 @@

     return vstart;

}

-#endif /* CONFIG_XEN_PHYSDEV_ACCESS */
+EXPORT_SYMBOL(allocate_empty_lowmem_region);
diff -r 2f743309f21a -r b235cde45efb
linux-2.6-xen-sparse/include/asm-xen/hypervisor.h

@@ -136,10 +136,8 @@
 
 void xen_contig_memory(unsigned long vstart, unsigned int order);
 
-#ifdef CONFIG_XEN_PHYSDEV_ACCESS
 /* Allocate a contiguous empty region of low memory. Return virtual start. */
 unsigned long allocate_empty_lowmem_region(unsigned long pages);
-#endif
 
 #include <asm/hypercall.h>

# HG changeset patch
# User [email protected]
# Node ID b235cde45efbb903d3e0e9626df6a90adde20577 # Parent 2f743309f21afdc6adaef01421da40f7ff1e8b0d Add allocate_empty_lowmem_region() for unprivileged domains

diff -r 2f743309f21a -r b235cde45efb linux-2.6-xen-sparse/arch/xen/i386/mm/hypervisor.c
@@ -35,6 +35,7 @@
 #include <asm/pgtable.h>
 #include <asm-xen/hypervisor.h>
 #include <asm-xen/balloon.h>
+#include <linux/module.h>
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,0)
 #include <linux/percpu.h>
 #include <asm/tlbflush.h>

@@ -312,7 +313,6 @@
     balloon_unlock(flags);
 }
 
-#ifdef CONFIG_XEN_PHYSDEV_ACCESS
 
 unsigned long allocate_empty_lowmem_region(unsigned long pages)
 {

@@ -360,4 +360,4 @@
     return vstart;
 }
 
-#endif /* CONFIG_XEN_PHYSDEV_ACCESS */
+EXPORT_SYMBOL(allocate_empty_lowmem_region);

diff -r 2f743309f21a -r b235cde45efb linux-2.6-xen-sparse/include/asm-xen/hypervisor.h
@@ -136,10 +136,8 @@
 
 void xen_contig_memory(unsigned long vstart, unsigned int order);
 
-#ifdef CONFIG_XEN_PHYSDEV_ACCESS
 /* Allocate a contiguous empty region of low memory. Return virtual start. */
 unsigned long allocate_empty_lowmem_region(unsigned long pages);
-#endif
 
 #include <asm/hypercall.h>
 

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I see you're adding an export so that it's usable by modules; I think that makes sense. Although the current backends must be statically compiled there's no reason not to export this for other backends to use. Harry tells me the 2.6 USB backend is fully modularised, for instance (he also pointed out there's no way to deallocate the empty lowmem region on unload, which'll need to be addressed at some stage).

I did wonder if we ought to tweak the name if we're exporting to the rest of the kernel (e.g. prefix xen_) but I don't think we have a convention for that.

Basically, +1 from me. I like it.

Cheers,
Mark