Re: [Patch v8 11/23] perf/x86: Enable XMM register sampling for REGS_USER case

[Peter Zijlstra]

On Fri, May 29, 2026 at 03:56:33PM +0800, Dapeng Mi wrote:
> This patch adds support for XMM register sampling in the REGS_USER case.
> 
> To handle simultaneous sampling of XMM registers for both REGS_INTR and
> REGS_USER cases, a per-CPU `x86_user_regs` is introduced to store
> REGS_USER-specific XMM registers. This prevents REGS_USER-specific XMM
> register data from being overwritten by REGS_INTR-specific data if they
> share the same `x86_perf_regs` structure.
> 
> To sample user-space XMM registers, the `x86_pmu_update_user_xregs()`
> helper function is added. It checks if the `TIF_NEED_FPU_LOAD` flag is
> set. If so, the user-space XMM register data can be directly retrieved
> from the cached task FPU state, as the corresponding hardware registers
> have been cleared or switched to kernel-space data. Otherwise, the data
> must be read from the hardware registers using the `xsaves` instruction.
> 
> For PEBS events, `x86_pmu_update_user_xregs()` checks if the PEBS-sampled
> XMM register data belongs to user-space. If so, no further action is
> needed. Otherwise, the user-space XMM register data needs to be
> re-sampled using the same method as for non-PEBS events.
> 
> Co-developed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>

Sashiko has fun comments; I don't think we care about the cross-vm data
leak, that's not worse than we already have on the regular regs. In
fact, it might be considered correct behaviour ;-)

It does have a point about noxsaves; or xsaves being masked by a VM.