On 22.04.25 11:12, Xin Li wrote:
On 4/22/2025 1:38 AM, Jürgen Groß wrote:On 22.04.25 10:21, Xin Li (Intel) wrote:To eliminate the indirect call overhead introduced by the pv_ops API, use the alternatives mechanism to read PMC:Which indirect call overhead? The indirect call is patched via the alternative mechanism to a direct one.See below.1) When built with !CONFIG_XEN_PV, X86_FEATURE_XENPV becomes a disabled feature, preventing the Xen PMC read code from being built and ensuring the native code is executed unconditionally.Without CONFIG_XEN_PV CONFIG_PARAVIRT_XXL is not selected, resulting in native code anyway.Yes, this is kept in this patch, but in a little different way.2) When built with CONFIG_XEN_PV: 2.1) If not running on the Xen hypervisor (!X86_FEATURE_XENPV), the kernel runtime binary is patched to unconditionally jump to the native PMC read code. 2.2) If running on the Xen hypervisor (X86_FEATURE_XENPV), the kernel runtime binary is patched to unconditionally jump to the Xen PMC read code. Consequently, remove the pv_ops PMC read API.I don't see the value of this patch. It adds more #ifdef and code lines without any real gain. In case the x86 maintainers think it is still worth it, I won't object.I think we want to totally bypass pv_ops in the case 2.1). Do you mean the indirect call is patched to call native code *directly* for 2.1? I don't know it, can you please elaborate?
All paravirt indirect calls are patched to direct calls via the normal alternative patch mechanism. Have a look at alt_replace_call() in arch/x86/kernel/alternative.c
AFAIK, Xen PV has been the sole user of pv_ops for nearly 20 years. This
Not quite. There was lguest until I ripped it out. :-) And some use cases are left for KVM and Hyper-V guests (I have kept those behind CONFIG_PARAVIRT, while the Xen-specific parts are behind CONFIG_PARAVIRT_XXL now).
raises significant doubts about whether pv_ops provides Linux with the value of being a well-abstracted "CPU" or "Platform". And the x86 maintainers have said that it's a maintenance nightmare.
I have worked rather hard to make it less intrusive, especially by removing the paravirt specific code patching (now all done via alternative patching) and by removing 32-bit Xen PV mode. Juergen
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