From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active Date: Mon, 10 Sep 2018 08:28:17 -0700 Message-ID: <1536593297.11460.72.camel@intel.com> References: <1536343050-18532-1-git-send-email-brijesh.singh@amd.com> <1536343050-18532-6-git-send-email-brijesh.singh@amd.com> <20180910122727.GE21815@zn.tnic> <026d5ca5-7b77-de6c-477e-ff39f0291ac0@amd.com> <1536586152.11460.40.camel@intel.com> <097eb5f5-2cd9-8b08-32c5-d90c8e0cbb6d@amd.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Cc: x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Tom Lendacky , Thomas Gleixner , "H. Peter Anvin" , Paolo Bonzini , Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= To: Brijesh Singh , Borislav Petkov Return-path: In-Reply-To: <097eb5f5-2cd9-8b08-32c5-d90c8e0cbb6d@amd.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Mon, 2018-09-10 at 10:10 -0500, Brijesh Singh wrote: > > On 09/10/2018 08:29 AM, Sean Christopherson wrote: > ... > > > > > > + */ > > > > > +static struct pvclock_vsyscall_time_info > > > > > + hv_clock_aux[NR_CPUS] __decrypted_aux; > > > > Hmm, so worst case that's 64 4K pages: > > > > > > > > (8192*32)/4096 = 64 4K pages. > > > We can minimize the worst case memory usage. The number of VCPUs > > > supported by KVM maybe less than NR_CPUS. e.g Currently KVM_MAX_VCPUS is > > > set to 288 > > KVM_MAX_VCPUS is a property of the host, whereas this code runs in the > > guest, e.g. KVM_MAX_VCPUS could be 2048 in the host for all we know. > > > > IIRC, during guest creation time qemu will check the host supported > VCPUS count. If count is greater than KVM_MAX_VCPUS then it will > fail to launch guest (or fail to hot plug vcpus). In other words, the > number of vcpus in a KVM guest will never to > KVM_MAX_VCPUS. > > Am I missing something ? KVM_MAX_VCPUS is a definition for use in the *host*, it's even defined in kvm_host.h.  The guest's pvclock code won't get magically recompiled if KVM_MAX_VCPUS is changed in the host.  KVM_MAX_VCPUS is an arbitrary value in the sense that there isn't a fundamental hard limit, i.e. the value can be changed, either for a custom KVM build or in mainline, e.g. it was bumped in 2016: commit 682f732ecf7396e9d6fe24d44738966699fae6c0 Author: Radim Krčmář Date:   Tue Jul 12 22:09:29 2016 +0200     KVM: x86: bump MAX_VCPUS to 288          288 is in high demand because of Knights Landing CPU.     We cannot set the limit to 640k, because that would be wasting space.          Reviewed-by: Paolo Bonzini     Signed-off-by: Radim Krčmář     Signed-off-by: Paolo Bonzini diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 074b5c760327..21a40dc7aad6 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -34,7 +34,7 @@  #include  #include   -#define KVM_MAX_VCPUS 255 +#define KVM_MAX_VCPUS 288  #define KVM_SOFT_MAX_VCPUS 240  #define KVM_USER_MEM_SLOTS 509  /* memory slots that are not exposed to userspace */