linux-kernel - Re: [PATCH] KVM: Switch 'requests' to be 64-bit (explicitly)

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Date:   Thu, 5 Jul 2018 14:51:39 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     KarimAllah Ahmed <karahmed@...zon.de>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: Switch 'requests' to be 64-bit (explicitly)

On Sun, Apr 15, 2018 at 12:26:44AM +0200, KarimAllah Ahmed wrote:
> Switch 'requests' to be explicitly 64-bit and update BUILD_BUG_ON check to
> use the size of "requests" instead of the hard-coded '32'.
> 
> That gives us a bit more room again for arch-specific requests as we
> already ran out of space for x86 due to the hard-coded check.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: kvm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> ---
>  include/linux/kvm_host.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6930c63..fe4f46b 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -129,7 +129,7 @@ static inline bool is_error_page(struct page *page)
>  #define KVM_REQUEST_ARCH_BASE     8
>  
>  #define KVM_ARCH_REQ_FLAGS(nr, flags) ({ \
> -	BUILD_BUG_ON((unsigned)(nr) >= 32 - KVM_REQUEST_ARCH_BASE); \
> +	BUILD_BUG_ON((unsigned)(nr) >= (sizeof(((struct kvm_vcpu *)0)->requests) * 8) - KVM_REQUEST_ARCH_BASE); \
>  	(unsigned)(((nr) + KVM_REQUEST_ARCH_BASE) | (flags)); \
>  })
>  #define KVM_ARCH_REQ(nr)           KVM_ARCH_REQ_FLAGS(nr, 0)
> @@ -223,7 +223,7 @@ struct kvm_vcpu {
>  	int vcpu_id;
>  	int srcu_idx;
>  	int mode;
> -	unsigned long requests;
> +	u64 requests;

The usual thing to do for bitmaps is something like:

#define KVM_REQUEST_NR		(KVM_REQUEST_ARCH_BASE + KVM_REQUEST_ARCH_NR)

	unsigned long requests[BITS_TO_LONGS(NR_KVM_REQUESTS)];

>  	unsigned long guest_debug;
>  
>  	int pre_pcpu;
> @@ -1122,7 +1122,7 @@ static inline void kvm_make_request(int req, struct kvm_vcpu *vcpu)
>  	 * caller.  Paired with the smp_mb__after_atomic in kvm_check_request.
>  	 */
>  	smp_wmb();
> -	set_bit(req & KVM_REQUEST_MASK, &vcpu->requests);
> +	set_bit(req & KVM_REQUEST_MASK, (void *)&vcpu->requests);

... which wouldn't require a void cast to make the bit API functions
happy (as these expect a pointer to the first unsigned long in the
bitmap).

Thanks,
Mark.