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

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Date:   Thu, 6 Sep 2018 06:50:41 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Brijesh Singh <brijesh.singh@....com>, 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 v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active

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().
> > 
> > Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> > Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > Cc: Tom Lendacky <thomas.lendacky@....com>
> > Cc: kvm@...r.kernel.org
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Borislav Petkov <bp@...e.de>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: linux-kernel@...r.kernel.org
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> > Cc: kvm@...r.kernel.org
> > Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> > ---
> >  arch/x86/include/asm/mem_encrypt.h |  4 ++++
> >  arch/x86/kernel/kvmclock.c         | 22 +++++++++++++++++++---
> >  arch/x86/kernel/vmlinux.lds.S      |  3 +++
> >  arch/x86/mm/init.c                 |  3 +++
> >  arch/x86/mm/mem_encrypt.c          | 10 ++++++++++
> >  5 files changed, 39 insertions(+), 3 deletions(-)
> > 
> > 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.

Originally, my thought was that this would be a one-off case and the
array could be freed directly in kvmclock_init(), e.g.:

static struct pvclock_vsyscall_time_info
	hv_clock_aux[HVC_AUX_ARRAY_SIZE] __decrypted __aligned(PAGE_SIZE);

...

void __init kvmclock_init(void)
{
	u8 flags;

	if (!sev_active())
		free_init_pages("unused decrypted",
			(unsigned long)hv_clock_aux,
			(unsigned long)hv_clock_aux + sizeof(hv_clock_aux));

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
> --