Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

[Sean Christopherson]

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.ÂKVM_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. ÂThe guest's pvclock code won't get magically recompiled
if KVM_MAX_VCPUS is changed in the host. ÂKVM_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 KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Date:ÂÂÂTue Jul 12 22:09:29 2016 +0200

ÂÂÂÂKVM: x86: bump MAX_VCPUS to 288
ÂÂÂÂ
ÂÂÂÂ288 is in high demand because of Knights Landing CPU.
ÂÂÂÂWe cannot set the limit to 640k, because that would be wasting space.
ÂÂÂÂ
ÂÂÂÂReviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
ÂÂÂÂSigned-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
ÂÂÂÂSigned-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

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 @@
Â#include <asm/asm.h>
Â#include <asm/kvm_page_track.h>
Â
-#define KVM_MAX_VCPUS 255
+#define KVM_MAX_VCPUS 288
Â#define KVM_SOFT_MAX_VCPUS 240
Â#define KVM_USER_MEM_SLOTS 509
Â/* memory slots that are not exposed to userspace */