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

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Date:   Mon, 10 Sep 2018 06:29:12 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>, Borislav Petkov <bp@...e.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock
 data when SEV is active

On Mon, 2018-09-10 at 08:15 -0500, Brijesh Singh wrote:
> 
> On 9/10/18 7:27 AM, Borislav Petkov wrote:
> > 
> > On Fri, Sep 07, 2018 at 12:57:30PM -0500, Brijesh Singh wrote:
> > > 
> > > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> > > index 376fd3a..6086b56 100644
> > > --- a/arch/x86/kernel/kvmclock.c
> > > +++ b/arch/x86/kernel/kvmclock.c
> > > @@ -65,6 +65,15 @@ static struct pvclock_vsyscall_time_info
> > >  static struct pvclock_wall_clock wall_clock __decrypted;
> > >  static DEFINE_PER_CPU(struct pvclock_vsyscall_time_info *, hv_clock_per_cpu);
> > >  
> > > +#ifdef CONFIG_AMD_MEM_ENCRYPT
> > > +/*
> > > + * The auxiliary array will be used when SEV is active. In non-SEV case,
> > > + * it will be freed by free_decrypted_mem().
> > > + */
> > > +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.

> (288 * 64)/4096 = 4 4K pages.
> 
> (pvclock_vsyscall_time_info is cache aligned so it will be 64 bytes)

Ah, I was wondering why my calculations were always different than
yours.  I was looking at struct pvclock_vcpu_time_info, which is 32
bytes.

> #if NR_CPUS > KVM_MAX_VCPUS
> #define HV_AUX_ARRAY_SIZE  KVM_MAX_VCPUS
> #else
> #define HV_AUX_ARRAY_SIZE NR_CPUS
> #endif
> 
> static struct pvclock_vsyscall_time_info
>                         hv_clock_aux[HV_AUX_ARRAY_SIZE] __decrypted_aux;