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

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Message-ID: <1531215466.10099.28.camel@amazon.de>
Date:   Tue, 10 Jul 2018 09:37:46 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     "mark.rutland@....com" <mark.rutland@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: Switch 'requests' to be 64-bit (explicitly)

On Thu, 2018-07-05 at 14:51 +0100, Mark Rutland wrote:
> 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).

Ah, right! Good point.

Paolo,

Would you prefer to switch "requests" to a bitmap and 
update kvm_request_pending to handle this?

> 
> Thanks,
> Mark.
> 
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