Re: [Xen-devel] [PATCH v2 2/3] x86/hyperv: skeleton for L0 assisted TLB flush

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 17, 2020 at 12:01:23PM +0000, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > Sent: 17 February 2020 11:41
> > To: Wei Liu <wl@xxxxxxx>
> > Cc: Durrant, Paul <pdurrant@xxxxxxxxxxxx>; Xen Development List <xen-
> > devel@xxxxxxxxxxxxxxxxxxxx>; Michael Kelley <mikelley@xxxxxxxxxxxxx>; Wei
> > Liu <liuwe@xxxxxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx>
> > Subject: Re: [PATCH v2 2/3] x86/hyperv: skeleton for L0 assisted TLB flush
> > 
> > On Mon, Feb 17, 2020 at 11:34:41AM +0000, Wei Liu wrote:
> > > On Fri, Feb 14, 2020 at 04:55:44PM +0000, Durrant, Paul wrote:
> > > > > -----Original Message-----
> > > > > From: Wei Liu <wei.liu.xen@xxxxxxxxx> On Behalf Of Wei Liu
> > > > > Sent: 14 February 2020 13:34
> > > > > To: Xen Development List <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> > > > > Cc: Michael Kelley <mikelley@xxxxxxxxxxxxx>; Durrant, Paul
> > > > > <pdurrant@xxxxxxxxxxxx>; Wei Liu <liuwe@xxxxxxxxxxxxx>; Wei Liu
> > > > > <wl@xxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Andrew Cooper
> > > > > <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > > > Subject: [PATCH v2 2/3] x86/hyperv: skeleton for L0 assisted TLB
> > flush
> > > > >
> > > > > Implement a basic hook for L0 assisted TLB flush. The hook needs to
> > > > > check if prerequisites are met. If they are not met, it returns an
> > error
> > > > > number to fall back to native flushes.
> > > > >
> > > > > Introduce a new variable to indicate if hypercall page is ready.
> > > > >
> > > > > Signed-off-by: Wei Liu <liuwe@xxxxxxxxxxxxx>
> > > > > ---
> > > > >  xen/arch/x86/guest/hyperv/Makefile  |  1 +
> > > > >  xen/arch/x86/guest/hyperv/hyperv.c  | 17 ++++++++++++
> > > > >  xen/arch/x86/guest/hyperv/private.h |  4 +++
> > > > >  xen/arch/x86/guest/hyperv/tlb.c     | 41
> > +++++++++++++++++++++++++++++
> > > > >  4 files changed, 63 insertions(+)
> > > > >  create mode 100644 xen/arch/x86/guest/hyperv/tlb.c
> > > > >
> > > > > diff --git a/xen/arch/x86/guest/hyperv/Makefile
> > > > > b/xen/arch/x86/guest/hyperv/Makefile
> > > > > index 68170109a9..18902c33e9 100644
> > > > > --- a/xen/arch/x86/guest/hyperv/Makefile
> > > > > +++ b/xen/arch/x86/guest/hyperv/Makefile
> > > > > @@ -1 +1,2 @@
> > > > >  obj-y += hyperv.o
> > > > > +obj-y += tlb.o
> > > > > diff --git a/xen/arch/x86/guest/hyperv/hyperv.c
> > > > > b/xen/arch/x86/guest/hyperv/hyperv.c
> > > > > index 70f4cd5ae0..f9d1f11ae3 100644
> > > > > --- a/xen/arch/x86/guest/hyperv/hyperv.c
> > > > > +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> > > > > @@ -33,6 +33,8 @@ DEFINE_PER_CPU_READ_MOSTLY(void *, hv_input_page);
> > > > >  DEFINE_PER_CPU_READ_MOSTLY(void *, hv_vp_assist);
> > > > >  DEFINE_PER_CPU_READ_MOSTLY(unsigned int, hv_vp_index);
> > > > >
> > > > > +static bool __read_mostly hv_hcall_page_ready;
> > > > > +
> > > > >  static uint64_t generate_guest_id(void)
> > > > >  {
> > > > >      union hv_guest_os_id id = {};
> > > > > @@ -119,6 +121,8 @@ static void __init setup_hypercall_page(void)
> > > > >      BUG_ON(!hypercall_msr.enable);
> > > > >
> > > > >      set_fixmap_x(FIX_X_HYPERV_HCALL, mfn << PAGE_SHIFT);
> > > >
> > > > Shouldn't this have at least a compiler barrier here?
> > > >
> > >
> > > OK. I will add a write barrier here.
> > 
> > Hm, shouldn't such barrier be part of set_fixmap_x itself?
> > 
> 
> Not really, for the purpose I had in mind. The hv_hcall_page_ready global is 
> specific to this code and we need to make sure the page is actually ready 
> before the code says it is.

But anything that modifies the page tables should already have a
barrier if required in order to prevent accesses from being moved
ahead of it, or else things would certainly go wrong in many other
places?

Roger.

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