Re: DESIGN: CPUID part 3

["Jan Beulich" <JBeulich@suse.com>]

>>> On 12.06.17 at 16:02, <andrew.cooper3@citrix.com> wrote:
> My original statement was "if the guest uses LBR/LER, then migration
> needs to be restricted to hardware with an identical LBR format".
> 
> You countered that, saying we could emulate LBR/LER as an alternative. 
> The implication here is that we could alter the LBR format via
> emulation, by cooking the value observed when the guest reads the LBR MSRs.
> 
> For the record, the formats are:
> 
> Software should query an architectural MSR IA32_PERF_CAPABILITIES[5:0]
> about the format of the address that is stored in the LBR stack. Four
> formats are defined by the following encoding:
> * 000000B (32-bit record format) — Stores 32-bit offset in current CS of
> respective source/destination,
> * 000001B (64-bit LIP record format) — Stores 64-bit linear address of
> respective source/destination,
> * 000010B (64-bit EIP record format) — Stores 64-bit offset (effective
> address) of respective source/destination.
> * 000011B (64-bit EIP record format) and Flags — Stores 64-bit offset
> (effective address) of respective source/destination. Misprediction info
> is reported in the upper bit of 'FROM' registers in the LBR stack. See
> LBR stack details below for flag support and definition.
> * 000100B (64-bit EIP record format), Flags and TSX — Stores 64-bit
> offset (effective address) of respective source/destination.
> Misprediction and TSX info are reported in the upper bits of ‘FROM’
> registers in the LBR stack.
> * 000101B (64-bit EIP record format), Flags, TSX, LBR_INFO — Stores
> 64-bit offset (effective address) of respective source/destination.
> Misprediction, TSX, and elapsed cycles since the last LBR update are
> reported in the LBR_INFO MSR stack.
> * 000110B (64-bit EIP record format), Flags, Cycles — Stores 64-bit
> linear address (CS.Base + effective address) of respective
> source/destination. Misprediction info is reported in the upper bits of
> 17-16 Vol. 3BDEBUG, BRANCH PROFILE, TSC, AND RESOURCE MONITORING
> FEATURES 'FROM' registers in the LBR stack. Elapsed cycles since the
> last LBR update are reported in the upper 16 bits of the 'TO' registers
> in the LBR stack (see Section 17.6).
> 
> In general, I don't see any sensible way of being able to convert
> between these formats at the point of an RDMSR.

Hmm, I don't see a problem converting formats 3..6 to formats 0
or 2. I also don't think any misbehavior can possibly result when
converting 2 to 3 by simply always loading a fixed value into the
mis-prediction bit. Whether 2 can be converted sensibly to 4..6
would need to be determined. Format 1 clearly is the odd one out,
conversion to/from which would only be reasonable if we assumed
flat addressing everywhere (which obviously we can assume as
long as a guest stays in 64-bit mode).

It is also clear that format 6 won't survive the addition of 5-level
page tables, as there aren't enough bits to store a meaningful
cycle count.

Jan

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