On 12/04/19 18:05, Igor Mammedov wrote: > Describe how to enable and detect modern CPU hotplug interface. > Detection part is based on new CPHP_GET_CPU_ID_CMD command, > introduced by "acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command" patch. > > Signed-off-by: Igor Mammedov <imamm...@redhat.com> > --- > docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-)
Could we make this usecase / workflow independent of the new CPHP_GET_CPU_ID_CMD command please? I'd like to suggest the following: > diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt > index bb33144..667b264 100644 > --- a/docs/specs/acpi_cpu_hotplug.txt > +++ b/docs/specs/acpi_cpu_hotplug.txt > @@ -15,14 +15,14 @@ CPU present bitmap for: > PIIX-PM (IO port 0xaf00-0xaf1f, 1-byte access) > One bit per CPU. Bit position reflects corresponding CPU APIC ID. > Read-only. > The first DWORD in bitmap is used in write mode to switch from legacy > - to new CPU hotplug interface, write 0 into it to do switch. > + to modern CPU hotplug interface, write 0 into it to do switch. > --------------------------------------------------------------- > QEMU sets corresponding CPU bit on hot-add event and issues SCI > with GPE.2 event set. CPU present map is read by ACPI BIOS GPE.2 handler > to notify OS about CPU hot-add events. CPU hot-remove isn't supported. > > ===================================== > -ACPI CPU hotplug interface registers: > +Modern ACPI CPU hotplug interface registers: > ------------------------------------- > Register block base address: > ICH9-LPC IO port 0x0cd8 > @@ -105,6 +105,24 @@ write access: > other values: reserved > > Typical usecases: > + - (x86) Detecting and enabling modern CPU hotplug interface. (1) I think we can drop the (x86) restriction. (Because, we don't need to depend on APIC ID specifics; see below.) > + QEMU starts with legacy CPU hotplug interface enabled. Detecting > and > + switching to modern interface is based on the 2 legacy CPU hotplug > features: > + 1. Writes into CPU bitmap are ignored. > + 2. CPU bitmap always has bit#0 set, corresponding to boot CPU. > + > + Use following steps to detect and enable modern CPU hotplug > interface: > + 1. Store 0x0 to the 'CPU selector' register, > + attempting to switch to modern mode > + 2. Store 0x0 to the 'CPU selector' register, > + to ensure valid selector value OK thus far. > + 3. Store 0x3 to the 'Command field' register, > + sets the 'Command data 2' register into architecture specific > + CPU identifier mode (2) Can we please store command 0 here (CPHP_GET_NEXT_CPU_WITH_EVENT_CMD)? That might change the selector value, yes. (Even if that happens, the new selector will be *valid*.) But the main point is that, with 0 stored to the command register, one of the following *four* cases will hold, subsequently: (2a) if register block is totally missing: --> Offset#0 will read as all-bits-one (unassigned read) (2b) if register block is legacy-only: --> Offset#0 will read as nonzero, due to CPU#0 being always present (2c) if the modern register block is active, but the "Command data 2" register is *not* yet described in the spec file: --> Offset#0 will read as zero, because it is *reserved*: > read access: > offset: > [0x0-0x3] reserved <---- HERE (2d) if the modern register block is active, and the "Command data 2" register *is* described in the spec file: --> the "Command data 2" register (at offset#0) will read as zero, because: > read access: > offset: > [0x0-0x3] Command data 2: (DWORD access) > if last stored 'Command field' value: > 3: upper 32 bits of architecture specific identifying CPU > value > (n x86 case: 0x0) > other values: reserved <------ HERE and then step#4 applies just the same: On 12/04/19 18:05, Igor Mammedov wrote: > + 4. Read the 'Command data 2' register. > + If read value is 0x0, the modern interface is enabled. > + Otherwise legacy or no CPU hotplug interface available > + because "read value is 0x0" corresponds to the *union* of cases (2c) and (2d) -- namely, "the modern register block is active". My proposal above is what I implemented for OVMF in October: [edk2-devel] [PATCH v2 3/3] OvmfPkg/PlatformPei: rewrite MaxCpuCountInitialization() for CPU hotplug http://mid.mail-archive.com/20191022221554.14963-4-lersek@redhat.com and it works very well. So the benefits would be: - I wouldn't have to rewrite that (complex!) patch :) , - the logic would not store the new CPHP_GET_CPU_ID_CMD command, only read offset#0, - the logic would not be x86 specific (it would not have to rely on the most significant 32 bits of the 64-bit arch-specific CPU identifier -- here: the APIC ID -- being zero). Furthermore, (3) In step#4, I suggest replacing 'Command data 2' with "offset 0", (4) finally, I'd suggest squashing this patch (updated as requested above) into patch "acpi: cpuhp: spec: add typical usecases". Using my suggestion (2), you can define the "modern interface enablement" workflow as well, without any dependency on CPHP_GET_CPU_ID_CMD. The only thing that's necessary is the small update from (3), and then you can describe all three important use cases in one go, in patch#6. And then you can introduce CPHP_GET_CPU_ID_CMD in the last patch (patch#7), while staying compatible with the previously-documented workflows. Pretty please? :) Thanks! Laszlo > - Get a cpu with pending event > 1. Store 0x0 to the 'CPU selector' register. > 2. Store 0x0 to the 'Command field' register. >
