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SubjectRe: [PATCH 1/5] KVM: SVM: Move msrpm offset calculation to seperate function
On 02/25/2010 07:15 PM, Joerg Roedel wrote:
> The algorithm to find the offset in the msrpm for a given
> msr is needed at other places too. Move that logic to its
> own function.
>
> #define MAX_INST_SIZE 15
>
> @@ -417,23 +439,22 @@ err_1:
> static void set_msr_interception(u32 *msrpm, unsigned msr,
> int read, int write)
> {
> - int i;
> + u8 bit_read, bit_write;
> + unsigned long tmp;
> + u32 offset;
>
> - for (i = 0; i< NUM_MSR_MAPS; i++) {
> - if (msr>= msrpm_ranges[i]&&
> - msr< msrpm_ranges[i] + MSRS_IN_RANGE) {
> - u32 msr_offset = (i * MSRS_IN_RANGE + msr -
> - msrpm_ranges[i]) * 2;
> -
> - u32 *base = msrpm + (msr_offset / 32);
> - u32 msr_shift = msr_offset % 32;
> - u32 mask = ((write) ? 0 : 2) | ((read) ? 0 : 1);
> - *base = (*base& ~(0x3<< msr_shift)) |
> - (mask<< msr_shift);
> - return;
> - }
> - }
> - BUG();
> + offset = svm_msrpm_offset(msr);
> + bit_read = 2 * (msr& 0x0f);
> + bit_write = 2 * (msr& 0x0f) + 1;
> +
> + BUG_ON(offset == MSR_INVALID);
> +
> + tmp = msrpm[offset];
> +
> + read ? clear_bit(bit_read,&tmp) : set_bit(bit_read,&tmp);
> + write ? clear_bit(bit_write,&tmp) : set_bit(bit_write,&tmp);
> +
> + msrpm[offset] = tmp;
> }
>

This can fault - set_bit() accesses an unsigned long, which can be 8
bytes, while offset can point into the last u32 of msrpm. So this needs
either to revert to u32 shift/mask ops or msrpm be changed to a ulong
array (actually better, since bitmaps in general are defined as arrays
of ulongs).

btw, the op-level ternary expression is terrible, relying solely on
*_bit()'s side effects. Please convert to an ordinary if.

btw2, use __set_bit() which atomic operation is not needed.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.



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