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

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

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?

confused,

greg k-h