Re: [Xen-devel] [PATCH] xen: allocate page for shared info page from low memory

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/23/2017 04:25 PM, Boris Ostrovsky wrote:
> On 06/14/2017 01:11 PM, Juergen Gross wrote:
> > On 14/06/17 18:58, Boris Ostrovsky wrote:
> > > On 06/12/2017 07:53 AM, Juergen Gross wrote:
> > > > In a HVM guest the kernel allocates the page for mapping the shared
> > > > info structure via extend_brk() today. This will lead to a drop of
> > > > performance as the underlying EPT entry will have to be split up into
> > > > 4kB entries as the single shared info page is located in hypervisor
> > > > memory.
> > > >
> > > > The issue has been detected by using the libmicro munmap test:
> > > > unmapping 8kB of memory was faster by nearly a factor of two when no
> > > > pv interfaces were active in the HVM guest.
> > > >
> > > > So instead of taking a page from memory which might be mapped via
> > > > large EPT entries use a page which is already mapped via a 4kB EPT
> > > > entry: we can take a page from the first 1MB of memory as the video
> > > > memory at 640kB disallows using larger EPT entries.
> > > >
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > > ---
> > > >   arch/x86/xen/enlighten_hvm.c | 31 ++++++++++++++++++++++++-------
> > > >   arch/x86/xen/enlighten_pv.c  |  2 --
> > > >   2 files changed, 24 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/arch/x86/xen/enlighten_hvm.c 
> > > > b/arch/x86/xen/enlighten_hvm.c
> > > > index a6d014f47e52..c19477b6e43a 100644
> > > > --- a/arch/x86/xen/enlighten_hvm.c
> > > > +++ b/arch/x86/xen/enlighten_hvm.c
> > > > @@ -1,5 +1,6 @@
> > > >   #include <linux/cpu.h>
> > > >   #include <linux/kexec.h>
> > > > +#include <linux/memblock.h>
> > > >   #include <xen/features.h>
> > > >   #include <xen/events.h>
> > > > @@ -10,9 +11,11 @@
> > > >   #include <asm/reboot.h>
> > > >   #include <asm/setup.h>
> > > >   #include <asm/hypervisor.h>
> > > > +#include <asm/e820/api.h>
> > > >   #include <asm/xen/cpuid.h>
> > > >   #include <asm/xen/hypervisor.h>
> > > > +#include <asm/xen/page.h>
> > > >   #include "xen-ops.h"
> > > >   #include "mmu.h"
> > > > @@ -22,20 +25,34 @@ void __ref xen_hvm_init_shared_info(void)
> > > >   {
> > > >       int cpu;
> > > >       struct xen_add_to_physmap xatp;
> > > > -    static struct shared_info *shared_info_page;
> > > > +    u64 pa;
> > > > +
> > > > +    if (HYPERVISOR_shared_info == &xen_dummy_shared_info) {
> > > > +        /*
> > > > +         * Search for a free page starting at 4kB physical address.
> > > > +         * Low memory is preferred to avoid an EPT large page split up
> > > > +         * by the mapping.
> > > > +         * Starting below X86_RESERVE_LOW (usually 64kB) is fine as
> > > > +         * the BIOS used for HVM guests is well behaved and won't
> > > > +         * clobber memory other than the first 4kB.
> > > > +         */
> > > > +        for (pa = PAGE_SIZE;
> > > > +             !e820__mapped_all(pa, pa + PAGE_SIZE, E820_TYPE_RAM) ||
> > > > +             memblock_is_reserved(pa);
> > > > +             pa += PAGE_SIZE)
> > > > +            ;
> > > Is it possible to never find a page here?
> > Only if there is no memory available at all. :-)
> >
> > TBH: I expect this to _always_ succeed at the first loop iteration.
> This patch seems to break (64-bit only) guests on dumpdata here. No 
> problems on other machines.
> So far all I know is that we did get the first page (0x1000) but not 
> much more. I will poke at this more on Monday.

So the problem is due to KASLR --- we can't use __va() before 
kernel_randomize_memory() is called since it will change __PAGE_OFFSET. 
(Setting CONFIG_RANDOMIZE_BASE will cause failure.)

-boris

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