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SubjectRe: [PATCH] adjust/fix LDT handling for Xen
Jan Beulich wrote:
> Don't rely on kmalloc(PAGE_SIZE) returning PAGE_SIZE aligned memory
> (Xen requires GDT *and* LDT to be page-aligned).

Can kmalloc return non-page-aligned PAGE_SIZE allocations?

> Using the page
> allocator interface also removes the (albeit small) slab allocator
> overhead.

Runtime or space overhead? Given that they're once-off allocations, the
time part isn't a big factor. And apparently LDT is completely unused.

Seems like a reasonable change either way.

> The same change being done for 64-bits for consistency.
>
> Further, the Xen hypercall interface expects the LDT address to be
> virtual, not machine.
>

Shows how much the LDT gets used then... Hm, but I didn't make that
code up; I wonder where it came from? But certainly, passing the mfn of
the first page of a vmalloc area doesn't make any sense.

> Signed-off-by: Jan Beulich <jbeulich@novell.com>
> Cc: Ingo Molnar <mingo@elte.hu>
>
Acked-by: Jeremy Fitzhardinge <jeremy@xensource.com>

> ---
> arch/x86/kernel/ldt_32.c | 7 +++----
> arch/x86/kernel/ldt_64.c | 7 +++----
> arch/x86/xen/enlighten.c | 9 +--------
> 3 files changed, 7 insertions(+), 16 deletions(-)
>
> --- linux-2.6.24-rc7/arch/x86/kernel/ldt_32.c 2008-01-10 16:53:54.000000000 +0100
> +++ 2.6.24-rc7-x86-xen-ldt/arch/x86/kernel/ldt_32.c 2008-01-09 13:59:50.000000000 +0100
> @@ -9,7 +9,6 @@
> #include <linux/mm.h>
> #include <linux/smp.h>
> #include <linux/vmalloc.h>
> -#include <linux/slab.h>
>
> #include <asm/uaccess.h>
> #include <asm/system.h>
> @@ -38,7 +37,7 @@ static int alloc_ldt(mm_context_t *pc, i
> if (mincount*LDT_ENTRY_SIZE > PAGE_SIZE)
> newldt = vmalloc(mincount*LDT_ENTRY_SIZE);
> else
> - newldt = kmalloc(mincount*LDT_ENTRY_SIZE, GFP_KERNEL);
> + newldt = (void *)__get_free_page(GFP_KERNEL);
>
> if (!newldt)
> return -ENOMEM;
> @@ -69,7 +68,7 @@ static int alloc_ldt(mm_context_t *pc, i
> if (oldsize*LDT_ENTRY_SIZE > PAGE_SIZE)
> vfree(oldldt);
> else
> - kfree(oldldt);
> + put_page(virt_to_page(oldldt));
> }
> return 0;
> }
> @@ -114,7 +113,7 @@ void destroy_context(struct mm_struct *m
> if (mm->context.size*LDT_ENTRY_SIZE > PAGE_SIZE)
> vfree(mm->context.ldt);
> else
> - kfree(mm->context.ldt);
> + put_page(virt_to_page(mm->context.ldt));
> mm->context.size = 0;
> }
> }
> --- linux-2.6.24-rc7/arch/x86/kernel/ldt_64.c 2008-01-10 16:53:54.000000000 +0100
> +++ 2.6.24-rc7-x86-xen-ldt/arch/x86/kernel/ldt_64.c 2008-01-09 13:59:50.000000000 +0100
> @@ -12,7 +12,6 @@
> #include <linux/mm.h>
> #include <linux/smp.h>
> #include <linux/vmalloc.h>
> -#include <linux/slab.h>
>
> #include <asm/uaccess.h>
> #include <asm/system.h>
> @@ -41,7 +40,7 @@ static int alloc_ldt(mm_context_t *pc, u
> if (mincount*LDT_ENTRY_SIZE > PAGE_SIZE)
> newldt = vmalloc(mincount*LDT_ENTRY_SIZE);
> else
> - newldt = kmalloc(mincount*LDT_ENTRY_SIZE, GFP_KERNEL);
> + newldt = (void *)__get_free_page(GFP_KERNEL);
>
> if (!newldt)
> return -ENOMEM;
> @@ -73,7 +72,7 @@ static int alloc_ldt(mm_context_t *pc, u
> if (oldsize*LDT_ENTRY_SIZE > PAGE_SIZE)
> vfree(oldldt);
> else
> - kfree(oldldt);
> + put_page(virt_to_page(oldldt));
> }
> return 0;
> }
> @@ -117,7 +116,7 @@ void destroy_context(struct mm_struct *m
> if ((unsigned)mm->context.size*LDT_ENTRY_SIZE > PAGE_SIZE)
> vfree(mm->context.ldt);
> else
> - kfree(mm->context.ldt);
> + put_page(virt_to_page(mm->context.ldt));
> mm->context.size = 0;
> }
> }
> --- linux-2.6.24-rc7/arch/x86/xen/enlighten.c 2008-01-10 16:53:55.000000000 +0100
> +++ 2.6.24-rc7-x86-xen-ldt/arch/x86/xen/enlighten.c 2008-01-09 14:00:51.000000000 +0100
> @@ -275,19 +275,12 @@ static unsigned long xen_store_tr(void)
>
> static void xen_set_ldt(const void *addr, unsigned entries)
> {
> - unsigned long linear_addr = (unsigned long)addr;
> struct mmuext_op *op;
> struct multicall_space mcs = xen_mc_entry(sizeof(*op));
>
> op = mcs.args;
> op->cmd = MMUEXT_SET_LDT;
> - if (linear_addr) {
> - /* ldt my be vmalloced, use arbitrary_virt_to_machine */
> - xmaddr_t maddr;
> - maddr = arbitrary_virt_to_machine((unsigned long)addr);
> - linear_addr = (unsigned long)maddr.maddr;
> - }
> - op->arg1.linear_addr = linear_addr;
> + op->arg1.linear_addr = (unsigned long)addr;
> op->arg2.nr_ents = entries;
>
> MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
>
>
>
>



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