linux-kernel - Re: [PATCH] kvm: change KVM_REQUEST_MASK to reflect vcpu.requests size

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Date:   Tue, 21 May 2019 20:28:36 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     kernel-team@...com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] kvm: change KVM_REQUEST_MASK to reflect vcpu.requests
 size

On 21/05/19 19:22, Rik van Riel wrote:
> The code using KVM_REQUEST_MASK uses a pattern reminiscent of a bitmask:
> 
> 	set_bit(req & KVM_REQUEST_MASK, &vcpu->requests);
> 
> However, the first argument passed to set_bit, test_bit, and clear_bit
> is a bit number, not a bitmask. That means the current definition would
> allow users of kvm_make_request to overflow the vcpu.requests bitmask,
> and is confusing to developers examining the code.

This is true, but the meaning of the masking is that bits above 7 define
extra things to do when sending a request (wait for acknowledge, kick
the recipient CPU).  The fact that the "request number" field is 8 bits
rather than 5 or 6 is just an implementation detail.

If you change it to BITS_PER_LONG-1, the obvious way to read the code
would be that requests 0, 64, 128 are all valid and map to the same request.

Paolo

> Redefine KVM_REQUEST_MASK to reflect the number of bits that actually
> fit inside an unsigned long, and add a comment explaining set_bit and
> friends take bit numbers, not a bitmask.