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SubjectRe: [PATCH] xen: allocate page for shared info page from low memory
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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@suse.com>
>>>> ---
>>>> 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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