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Date: Thu, 25 Apr 2019 13:00:25 +0200 From: Borislav Petkov <bp@...en8.de> To: "Zhao, Yakui" <yakui.zhao@...el.com> Cc: Ingo Molnar <mingo@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, "tglx@...utronix.de" <tglx@...utronix.de>, "Chen, Jason CJ" <jason.cj.chen@...el.com> Subject: Re: [RFC PATCH v5 4/4] x86/acrn: Add hypercall for ACRN guest On Thu, Apr 25, 2019 at 06:16:02PM +0800, Zhao, Yakui wrote: > The parameter register for the VMCALL is predefined in ACRN hypervisor. Now > the R8 is used to pass the hcall_id. > It seems that there is no special constraint for R8~R15. > So the explicit register variable is used so that the R8 can be passed. If you're going to use the constraint "D" for param1, you can just as well do "=a" (result) everywhere since you have the letter constraint for %rax instead of declaring it with "register". Also, you can completely get rid of those "register" declarations and let gcc have all the freedom to pass in hcall_id and the other parameters: unsigned long result; asm volatile("mov %[hcall_id], %%r8\n\t" "vmcall\n\t" : "=a" (result) : [hcall_id] "g" (hcall_id) : "r8"); return result; and %r8 will be in the clobber list so gcc will reload it if needed. gcc turns it into 0000000000001040 <main>: 1040: 4c 8b 05 e1 2f 00 00 mov 0x2fe1(%rip),%r8 # 4028 <hcall_id> 1047: 0f 01 c1 vmcall 104a: c3 retq 104b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) here. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
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