[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xendevel] [PATCH 00/13] Add VMX TSC scaling support
On 11/24/15 21:05, Haozhong Zhang wrote: [...] > > > > > > This bug can be fixed either later by patch 5 which introduces a > > > common function hvm_scale_tsc() to scale TSC, or by replacing above > > > underlined code with a simplified and inlined version of > > > hvm_scale_tsc() as below: > > > uint64_t mult, frac; > > > mult = ratio >> 32; > > > frac = ratio & ((1ULL << 32)  1); > > > offset = host_tsc * mult; > > > offset += (host_tsc >> 32) * frac; > > > offset += ((host_tsc & ((1ULL << 32)  1)) * frac) >> 32; > > > > I am not sure I understand the last line (or maybe 2 lines) > > > > Just simple math with carefulness to avoid 64bit integer overflow: > > suppose the most significant 32 bits of host_tsc and ratio are tsc_h > and mult, and the least significant 32 bits of them are tsc_l and > frac, then > host_tsc * ratio * 2^32 > = host_tsc * (mult * 2^32 + frac) * 2^32 > = host_tsc * mult + (tsc_h * 2^32 + tsc_l) * frac * 2^32 > = host_tsc * mult + tsc_h * frac + ((tsc_l * frac) >> 32) > > All multiplications in the last line are between 32bit integers, so none > of them could overflow 64bit integers. Sorry, it should be "All but the first multiplication host_tsc * mult ...". In practice, it should be very rare for the first term to overflow (considering mult is usually less than 10). Haozhong > > Consider a simple example that host_tsc = 1ULL << 33 and ratio = 0xffffffff. > Overflow happens in the multiplication of the second term of your formula > below, > and all overflowed bits are lost in the next right shift. > > Haozhong > > > If by 'offset' here you are trying to calculate the scaled version of host > > TSC then I think it would be > > > > (host_tsc * (ratio >> 32)) + ( (host_tsc * (ratio & 0xffffffff)) >> 32 ) > > > > (sanity check: assuming host_tsc is 8 and the ratio is 1.5 (i.e. > > 0x180000000) we get 12) > > > > > > boris _______________________________________________ Xendevel mailing list Xendevel@xxxxxxxxxxxxx http://lists.xen.org/xendevel

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