linux-kernel - Re: [PATCH RESEND v4 2/2] arm/arm64: KVM: Add KVM_GET/SET_VCPU

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Date:   Sun, 17 Jun 2018 01:30:20 +0000
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     James Morse <james.morse@....com>
CC:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "christoffer.dall@....com" <christoffer.dall@....com>,
        "marc.zyngier@....com" <marc.zyngier@....com>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH RESEND v4 2/2] arm/arm64: KVM: Add
 KVM_GET/SET_VCPU_EVENTS

> >>>>> --- a/arch/arm/include/uapi/asm/kvm.h
> >>>>> +++ b/arch/arm/include/uapi/asm/kvm.h
> >>>>> @@ -124,6 +124,18 @@ struct kvm_sync_regs {  struct
> >>>>> kvm_arch_memory_slot {  };
> >>>>>
> >>>>> +/* for KVM_GET/SET_VCPU_EVENTS */ struct kvm_vcpu_events {
> >>>>> +	struct {
> >>>>> +		__u8 serror_pending;
> >>>>> +		__u8 serror_has_esr;
> >>>>> +		/* Align it to 8 bytes */
> >>>>> +		__u8 pad[6];
> >>>>> +		__u64 serror_esr;
> >>>>> +	} exception;
> >>>>> +	__u32 reserved[12];
> >>>>> +};
> >>>>> +
> >>>>
> >>>> You haven't defined __KVM_HAVE_VCPU_EVENTS for 32bit, so presumably
> >>>> this struct will never be used. Why is it here?
> >>
> >>>   if not add it for 32 bits. the 32 arm platform will build Fail, whether you have good
> >>>    idea to avoid this Failure if not add this struct for the 32 bit?
> >>
> >> How does this 32bit code build without this patch?
> >> If do you provide the struct, how will that code build with older headers?
> >>
> >> As far as I can see, this is what the __KVM_HAVE_VCPU_EVENTS define is for.
> >>
> >> This should be both, or neither. Having just the struct is useless.
> > It because the caller of kvm_arm_vcpu_get/set_events() is in "virt/kvm/arm/arm.c".
> > the virt/kvm/arm/arm.c will used by both arm64 and arm.
> > so It needs to add kvm_arm_vcpu_get/set_events() for the 32 bits,
> > however, kvm_arm_vcpu_get/set_events() will directly return,
> 
> So you are adding a uapi struct that user-space can't actually use, to avoid a kernel build-error. Fine, it just looks really strange.
> 
> 32bit user-space shouldn't try to call this as check-extension reports it as not present. If it does, it gets -EINVAL back, which is also the
> default for kvm_arch_vcpu_ioctl().

It indeed looks strange. Because "virt/kvm/arm/arm.c" is shared by arm64 and arm32, if not adding, it will build-error. Do you have a better idea to avoid adding in arm32? 
I still not think out a good method. 32 user-space will not call this KVM_GET/SET_VCPU_EVENTS IOCTL as check-extension reports it as not present.

> 
> 
> Thanks,
> 
> James