Re: [RFC 02/16] x86/kvm: Introduce KVM memory protection feature

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, May 25, 2020 at 04:58:51PM +0200, Vitaly Kuznetsov wrote:
> "Kirill A. Shutemov" <kirill@shutemov.name> writes:
> 
> > Provide basic helpers, KVM_FEATURE and a hypercall.
> >
> > Host side doesn't provide the feature yet, so it is a dead code for now.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  arch/x86/include/asm/kvm_para.h      |  5 +++++
> >  arch/x86/include/uapi/asm/kvm_para.h |  3 ++-
> >  arch/x86/kernel/kvm.c                | 16 ++++++++++++++++
> >  include/uapi/linux/kvm_para.h        |  3 ++-
> >  4 files changed, 25 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> > index 9b4df6eaa11a..3ce84fc07144 100644
> > --- a/arch/x86/include/asm/kvm_para.h
> > +++ b/arch/x86/include/asm/kvm_para.h
> > @@ -10,11 +10,16 @@ extern void kvmclock_init(void);
> >  
> >  #ifdef CONFIG_KVM_GUEST
> >  bool kvm_check_and_clear_guest_paused(void);
> > +bool kvm_mem_protected(void);
> >  #else
> >  static inline bool kvm_check_and_clear_guest_paused(void)
> >  {
> >  	return false;
> >  }
> > +static inline bool kvm_mem_protected(void)
> > +{
> > +	return false;
> > +}
> >  #endif /* CONFIG_KVM_GUEST */
> >  
> >  #define KVM_HYPERCALL \
> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> > index 2a8e0b6b9805..c3b499acc98f 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -28,9 +28,10 @@
> >  #define KVM_FEATURE_PV_UNHALT		7
> >  #define KVM_FEATURE_PV_TLB_FLUSH	9
> >  #define KVM_FEATURE_ASYNC_PF_VMEXIT	10
> > -#define KVM_FEATURE_PV_SEND_IPI	11
> > +#define KVM_FEATURE_PV_SEND_IPI		11
> 
> Nit: spurrious change
> 

I fixed indentation while there. (Look at the file, not the diff to see
what I mean).

> >  #define KVM_FEATURE_POLL_CONTROL	12
> >  #define KVM_FEATURE_PV_SCHED_YIELD	13
> > +#define KVM_FEATURE_MEM_PROTECTED	14
> >  
> >  #define KVM_HINTS_REALTIME      0
> >  
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 6efe0410fb72..bda761ca0d26 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -35,6 +35,13 @@
> >  #include <asm/tlb.h>
> >  #include <asm/cpuidle_haltpoll.h>
> >  
> > +static bool mem_protected;
> > +
> > +bool kvm_mem_protected(void)
> > +{
> > +	return mem_protected;
> > +}
> > +
> 
> Honestly, I don't see a need for kvm_mem_protected(), just rename the
> bool if you need kvm_ prefix :-)

For !CONFIG_KVM_GUEST it would not be a variable. We may want to change it
to static branch or something in the future.

> >  static int kvmapf = 1;
> >  
> >  static int __init parse_no_kvmapf(char *arg)
> > @@ -727,6 +734,15 @@ static void __init kvm_init_platform(void)
> >  {
> >  	kvmclock_init();
> >  	x86_platform.apic_post_init = kvm_apic_init;
> > +
> > +	if (kvm_para_has_feature(KVM_FEATURE_MEM_PROTECTED)) {
> > +		if (kvm_hypercall0(KVM_HC_ENABLE_MEM_PROTECTED)) {
> > +			pr_err("Failed to enable KVM memory protection\n");
> > +			return;
> > +		}
> > +
> > +		mem_protected = true;
> > +	}
> >  }
> 
> Personally, I'd prefer to do this via setting a bit in a KVM-specific
> MSR instead. The benefit is that the guest doesn't need to remember if
> it enabled the feature or not, it can always read the config msr. May
> come handy for e.g. kexec/kdump.

I think we would need to remember it anyway. Accessing MSR is somewhat
expensive. But, okay, I can rework it MSR if needed.

Note, that we can avoid the enabling algother, if we modify BIOS to deal
with private/shared memory. Currently BIOS get system crash if we enable
the feature from time zero.

> >  const __initconst struct hypervisor_x86 x86_hyper_kvm = {
> > diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h
> > index 8b86609849b9..1a216f32e572 100644
> > --- a/include/uapi/linux/kvm_para.h
> > +++ b/include/uapi/linux/kvm_para.h
> > @@ -27,8 +27,9 @@
> >  #define KVM_HC_MIPS_EXIT_VM		7
> >  #define KVM_HC_MIPS_CONSOLE_OUTPUT	8
> >  #define KVM_HC_CLOCK_PAIRING		9
> > -#define KVM_HC_SEND_IPI		10
> > +#define KVM_HC_SEND_IPI			10
> 
> Same spurrious change detected.

The same justification :)

> >  #define KVM_HC_SCHED_YIELD		11
> > +#define KVM_HC_ENABLE_MEM_PROTECTED	12
> >  
> >  /*
> >   * hypercalls use architecture specific
> 
> -- 
> Vitaly
> 
> 

-- 
 Kirill A. Shutemov