RE: [PATCH v10 2/7] arm64: hyperv: Add Hyper-V hypercall and register access utilities

[Michael Kelley <mikelley@microsoft.com>]

From: Mark Rutland <mark.rutland@arm.com> Sent: Monday, May 17, 2021 4:45 AM
> 
> On Fri, May 14, 2021 at 03:14:41PM +0000, Michael Kelley wrote:
> > From: Mark Rutland <mark.rutland@arm.com> Sent: Friday, May 14, 2021 5:53 AM
> > >
> > > On Wed, May 12, 2021 at 10:37:42AM -0700, Michael Kelley wrote:
> > > > hyperv-tlfs.h defines Hyper-V interfaces from the Hyper-V Top Level
> > > > Functional Spec (TLFS), and #includes the architecture-independent
> > > > part of hyperv-tlfs.h in include/asm-generic.  The published TLFS
> > > > is distinctly oriented to x86/x64, so the ARM64-specific
> > > > hyperv-tlfs.h includes information for ARM64 that is not yet formally
> > > > published. The TLFS is available here:
> > > >
> > > >   docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs
> > > >
> > > > mshyperv.h defines Linux-specific structures and routines for
> > > > interacting with Hyper-V on ARM64, and #includes the architecture-
> > > > independent part of mshyperv.h in include/asm-generic.
> > > >
> > > > Use these definitions to provide utility functions to make
> > > > Hyper-V hypercalls and to get and set Hyper-V provided
> > > > registers associated with a virtual processor.
> > > >
> > > > Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> > > > Reviewed-by: Sunil Muthuswamy <sunilmut@microsoft.com>
> > > > ---
> > > >  MAINTAINERS                          |   3 +
> > > >  arch/arm64/Kbuild                    |   1 +
> > > >  arch/arm64/hyperv/Makefile           |   2 +
> > > >  arch/arm64/hyperv/hv_core.c          | 130
> +++++++++++++++++++++++++++++++++++
> > > >  arch/arm64/include/asm/hyperv-tlfs.h |  69 +++++++++++++++++++
> > > >  arch/arm64/include/asm/mshyperv.h    |  54 +++++++++++++++
> > > >  6 files changed, 259 insertions(+)
> > > >  create mode 100644 arch/arm64/hyperv/Makefile
> > > >  create mode 100644 arch/arm64/hyperv/hv_core.c
> > > >  create mode 100644 arch/arm64/include/asm/hyperv-tlfs.h
> > > >  create mode 100644 arch/arm64/include/asm/mshyperv.h
> > >
> > > > +/*
> > > > + * hv_do_hypercall- Invoke the specified hypercall
> > > > + */
> > > > +u64 hv_do_hypercall(u64 control, void *input, void *output)
> > > > +{
> > > > +	struct arm_smccc_res	res;
> > > > +	u64			input_address;
> > > > +	u64			output_address;
> > > > +
> > > > +	input_address = input ? virt_to_phys(input) : 0;
> > > > +	output_address = output ? virt_to_phys(output) : 0;
> > >
> > > I may have asked this before, but are `input` and `output` always linear
> > > map pointers, or can they ever be vmalloc pointers?
> > >
> > > Otherwise, this looks fine to me.
> >
> > The caller must ensure that hypercall arguments are aligned to
> > 4 Kbytes, and no larger than 4 Kbytes, since that's the page size
> > used by Hyper-V regardless of the guest page size.  A per-CPU
> > 4 Kbyte memory area (hyperv_pcpu_input_arg) meeting these
> > requirements is pre-allocated that callers can use for this purpose.
> 
> What I was trying to find out was how that was allocated, as vmalloc()'d
> pointers aren't legitimate to pass to virt_to_phys().
> 
> From scanning ahead to patch 5, I see that memory comes from kmalloc(),
> and so it is legitimate to use virt_to_phys().
> 
> 
> I see; and from patch 5 I see that memory come from kmalloc(), and will
> therefore be part of the linear map, and so virt_to_phys() is
> legitimate.
> 
> What I was asking here was how that memory was allocated. So long as
> those are the only buffers used, this looks fine to me.
> 

There is no vmalloc() or stack local memory used as arguments to a
hypercall.   Call sites know about this requirement along with the
requirement of being aligned to 4 Kbytes and no larger than 4 Kbytes.

Michael