Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Sep 06, 2018 at 06:50:41AM -0700, Sean Christopherson wrote:
> On Thu, Sep 06, 2018 at 02:24:23PM +0200, Borislav Petkov wrote:
> > On Thu, Sep 06, 2018 at 06:43:02AM -0500, Brijesh Singh wrote:
> > > Currently, the per-cpu pvclock data is allocated dynamically when
> > > cpu > HVC_BOOT_ARRAY_SIZE. The physical address of this variable is
> > > shared between the guest and the hypervisor hence it must be mapped as
> > > unencrypted (ie. C=0) when SEV is active.
> > > 
> > > When SEV is active, we will be wasting fairly sizeable amount of memory
> > > since each CPU will be doing a separate 4k allocation so that it can clear
> > > C-bit. Let's define few extra static page sized array of pvclock data.
> > > In the preparatory stage of CPU hotplug, use the element of this static
> > > array to avoid the dynamic allocation. This array will be put in
> > > the .data..decrypted section so that its mapped with C=0 during the boot.
> > > 
> > > In non-SEV case, this static page will unused and free'd by the
> > > free_decrypted_mem().
> > > 
> > > diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> > > index 802b2eb..aa204af 100644
> > > --- a/arch/x86/include/asm/mem_encrypt.h
> > > +++ b/arch/x86/include/asm/mem_encrypt.h
> > > @@ -48,11 +48,13 @@ int __init early_set_memory_encrypted(unsigned long vaddr, unsigned long size);
> > >  
> > >  /* Architecture __weak replacement functions */
> > >  void __init mem_encrypt_init(void);
> > > +void __init free_decrypted_mem(void);
> > >  
> > >  bool sme_active(void);
> > >  bool sev_active(void);
> > >  
> > >  #define __decrypted __attribute__((__section__(".data..decrypted")))
> > > +#define __decrypted_hvclock __attribute__((__section__(".data..decrypted_hvclock")))
> > 
> > So are we going to be defining a decrypted section for every piece of
> > machinery now?
> > 
> > That's a bit too much in my book.
> > 
> > Why can't you simply free everything in .data..decrypted on !SVE guests?
> 
> That would prevent adding __decrypted to existing declarations, e.g.
> hv_clock_boot, which would be ugly in its own right.  A more generic
> solution would be to add something like __decrypted_exclusive to mark
> data that is used if and only if SEV is active, and then free the
> SEV-only data when SEV is disabled.

Oh, and we'd need to make sure __decrypted_exclusive is freed when
!CONFIG_AMD_MEM_ENCRYPT, and preferably !sev_active() since the big
array is used only if SEV is active.  This patch unconditionally
defines hv_clock_dec but only frees it if CONFIG_AMD_MEM_ENCRYPT=y &&
!mem_encrypt_active().