Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752346AbdFNQ61 (ORCPT ); Wed, 14 Jun 2017 12:58:27 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:29599 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751714AbdFNQ60 (ORCPT ); Wed, 14 Jun 2017 12:58:26 -0400 Subject: Re: [PATCH] xen: allocate page for shared info page from low memory To: Juergen Gross , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org References: <20170612115356.8312-1-jgross@suse.com> From: Boris Ostrovsky Message-ID: Date: Wed, 14 Jun 2017 12:58:10 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170612115356.8312-1-jgross@suse.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3607 Lines: 105 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 > --- > 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 > #include > +#include > > #include > #include > @@ -10,9 +11,11 @@ > #include > #include > #include > +#include > > #include > #include > +#include > > #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? -boris > + > + memblock_reserve(pa, PAGE_SIZE); > + HYPERVISOR_shared_info = __va(pa); > + } > > - if (!shared_info_page) > - shared_info_page = (struct shared_info *) > - extend_brk(PAGE_SIZE, PAGE_SIZE); > xatp.domid = DOMID_SELF; > xatp.idx = 0; > xatp.space = XENMAPSPACE_shared_info; > - xatp.gpfn = __pa(shared_info_page) >> PAGE_SHIFT; > + xatp.gpfn = virt_to_pfn(HYPERVISOR_shared_info); > if (HYPERVISOR_memory_op(XENMEM_add_to_physmap, &xatp)) > BUG(); > > - HYPERVISOR_shared_info = (struct shared_info *)shared_info_page; > - > /* xen_vcpu is a pointer to the vcpu_info struct in the shared_info > * page, we use it in the event channel upcall and in some pvclock > * related functions. We don't need the vcpu_info placement > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c > index f33eef4ebd12..a9a67ecf2c07 100644 > --- a/arch/x86/xen/enlighten_pv.c > +++ b/arch/x86/xen/enlighten_pv.c > @@ -89,8 +89,6 @@ > > void *xen_initial_gdt; > > -RESERVE_BRK(shared_info_page_brk, PAGE_SIZE); > - > static int xen_cpu_up_prepare_pv(unsigned int cpu); > static int xen_cpu_dead_pv(unsigned int cpu); >