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SubjectRe: [PATCH] xen: allocate page for shared info page from low memory
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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.


Juergen

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