Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sun, Sep 27, 2020 at 12:51:52PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Sep 22, 2020 at 07:42:58PM +0800, shuo.a.liu@intel.com wrote:
> > From: Shuo Liu <shuo.a.liu@intel.com>
> > 
> > The Service VM communicates with the hypervisor via conventional
> > hypercalls. VMCALL instruction is used to make the hypercalls.
> > 
> > ACRN hypercall ABI:
> >   * Hypercall number is in R8 register.
> >   * Up to 2 parameters are in RDI and RSI registers.
> >   * Return value is in RAX register.
> > 
> > Introduce the ACRN hypercall interfaces. Because GCC doesn't support R8
> > register as direct register constraints, here are two ways to use R8 in
> > extended asm:
> >   1) use explicit register variable as input
> >   2) use supported constraint as input with a explicit MOV to R8 in
> >      beginning of asm
> > 
> > The number of instructions of above two ways are same.
> > Asm code from 1)
> >   38:   41 b8 00 00 00 80       mov    $0x80000000,%r8d
> >   3e:   48 89 c7                mov    %rax,%rdi
> >   41:   0f 01 c1                vmcall
> > Here, writes to the lower dword (%r8d) clear the upper dword of %r8 when
> > the CPU is in 64-bit mode.
> > 
> > Asm code from 2)
> >   38:   48 89 c7                mov    %rax,%rdi
> >   3b:   49 b8 00 00 00 80 00    movabs $0x80000000,%r8
> >   42:   00 00 00
> >   45:   0f 01 c1                vmcall
> > 
> > Choose 1) for code simplicity and a little bit of code size
> > optimization.
> > 
> > Originally-by: Yakui Zhao <yakui.zhao@intel.com>
> > Signed-off-by: Shuo Liu <shuo.a.liu@intel.com>
> > Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
> > Cc: Dave Hansen <dave.hansen@intel.com>
> > Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Fengwei Yin <fengwei.yin@intel.com>
> > Cc: Zhi Wang <zhi.a.wang@intel.com>
> > Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
> > Cc: Yu Wang <yu1.wang@intel.com>
> > Cc: Reinette Chatre <reinette.chatre@intel.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > ---
> >  arch/x86/include/asm/acrn.h | 57 +++++++++++++++++++++++++++++++++++++
> >  1 file changed, 57 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/acrn.h b/arch/x86/include/asm/acrn.h
> > index a2d4aea3a80d..23a93b87edeb 100644
> > --- a/arch/x86/include/asm/acrn.h
> > +++ b/arch/x86/include/asm/acrn.h
> > @@ -14,4 +14,61 @@ void acrn_setup_intr_handler(void (*handler)(void));
> >  void acrn_remove_intr_handler(void);
> >  bool acrn_is_privileged_vm(void);
> >  
> > +/*
> > + * Hypercalls for ACRN
> > + *
> > + * - VMCALL instruction is used to implement ACRN hypercalls.
> > + * - ACRN hypercall ABI:
> > + *   - Hypercall number is passed in R8 register.
> > + *   - Up to 2 arguments are passed in RDI, RSI.
> > + *   - Return value will be placed in RAX.
> > + */
> > +static inline long acrn_hypercall0(unsigned long hcall_id)
> > +{
> > +	register long r8 asm("r8");
> > +	long result;
> > +
> > +	/* Nothing can come between the r8 assignment and the asm: */
> > +	r8 = hcall_id;
> > +	asm volatile("vmcall\n\t"
> > +		     : "=a" (result)
> > +		     : "r" (r8)
> > +		     : );
> 
> What keeps an interrupt from happening between the r8 assignment and the
> asm: ?
> 
> Is this something that most hypercalls need to handle?  I don't see
> other ones needing this type of thing, is it just because of how these
> are defined?

Ah, the changelog above explains this.  You should put that in the code
itself, as a comment, otherwise we will not know this at all in 5
years, when gcc is changed to allow r8 access :)

thanks,

greg k-h