On Mon, 04 Mar 2024 19:10:08 +0000, Mark Brown <broonie@xxxxxxxxxx> wrote: > > [1 <text/plain; us-ascii (quoted-printable)>] > On Sat, Mar 02, 2024 at 11:19:35AM +0000, Marc Zyngier wrote: > > In retrospect, it is fairly obvious that the FP state ownership > > is only meaningful for a given CPU, and that locating this > > information in the vcpu was just a mistake. > > > > Move the ownership tracking into the host data structure, and > > rename it from fp_state to fp_owner, which is a better description > > (name suggested by Mark Brown). > > The SME patch series proposes adding an additional state to this > enumeration which would say if the registers are stored in a format > suitable for exchange with userspace, that would make this state part of > the vCPU state. With the addition of SME we can have two vector lengths > in play so the series proposes picking the larger to be the format for > userspace registers. What does this addition have anything to do with the ownership of the physical register file? Not a lot, it seems. Specially as there better be no state resident on the CPU when userspace messes up with it. > > We could store this separately to fp_state/owner but it'd still be a > value stored in the vCPU. I totally disagree. > Storing in a format suitable for userspace > usage all the time when we've got SME would most likely result in > performance overhead What performance overhead? Why should we care? > if nothing else and feels more complicated than > rewriting the data in the relatively unusual case where userspace looks > at it. Trying to convert userspace writes into the current layout would > have issues if the current layout uses the smaller vector length and > create fragility with ordering issues when loading the guest state. What ordering issues? If userspace manipulates the guest state, the guest isn't running. If it is, all bets are off. > > The proposal is not the most lovely idea ever but given the architecture > I think some degree of clunkiness would be unavoidable. It is only unavoidable if we decide to make a bad job of it. M. -- Without deviation from the norm, progress is not possible.
