| Message ID | 20190521132200.2b45c029@imladris.surriel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | kvm: change KVM_REQUEST_MASK to reflect vcpu.requests size | expand |
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.
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 79fa4426509c..d15fb43d7796 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -138,7 +138,8 @@ static inline bool is_error_page(struct page *page) return IS_ERR(page); } -#define KVM_REQUEST_MASK GENMASK(7,0) +/* Limit the bit numbers for set_bit etc to fit unsigned long vcpu.requests. */ +#define KVM_REQUEST_MASK (BITS_PER_LONG-1) #define KVM_REQUEST_NO_WAKEUP BIT(8) #define KVM_REQUEST_WAIT BIT(9) /*
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. 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. Signed-off-by: Rik van Riel <riel@surriel.com> --- include/linux/kvm_host.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)