Re: [PATCH linux v3 3/9] xen: introduce xen_vcpu_id mapping

[Vitaly Kuznetsov]

Stefano Stabellini <sstabellini@xxxxxxxxxx> writes:

> On Tue, 6 Sep 2016, Vitaly Kuznetsov wrote:
>> Stefano Stabellini <sstabellini@xxxxxxxxxx> writes:
>> 
>> > On Mon, 5 Sep 2016, Vitaly Kuznetsov wrote:
>> >> Julien Grall <julien.grall@xxxxxxx> writes:
>> >> 
>> >> > Hi Vitaly,
>> >> >
>> >> > On 26/07/16 13:30, Vitaly Kuznetsov wrote:
>> >> >> It may happen that Xen's and Linux's ideas of vCPU id diverge. In
>> >> >> particular, when we crash on a secondary vCPU we may want to do kdump
>> >> >> and unlike plain kexec where we do migrate_to_reboot_cpu() we try booting
>> >> >> on the vCPU which crashed. This doesn't work very well for PVHVM guests as
>> >> >> we have a number of hypercalls where we pass vCPU id as a parameter. These
>> >> >> hypercalls either fail or do something unexpected. To solve the issue
>> >> >> introduce percpu xen_vcpu_id mapping. ARM and PV guests get direct mapping
>> >> >> for now. Boot CPU for PVHVM guest gets its id from CPUID. With secondary
>> >> >> CPUs it is a bit more trickier. Currently, we initialize IPI vectors
>> >> >> before these CPUs boot so we can't use CPUID. Use ACPI ids from MADT
>> >> >> instead.
>> >> >>
>> >> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> >> >> ---
>> >> >> Changes since v2:
>> >> >> - Use uint32_t for xen_vcpu_id mapping [Julien Grall]
>> >> >>
>> >> >> Changes since v1:
>> >> >> - Introduce xen_vcpu_nr() helper [David Vrabel]
>> >> >> - Use ACPI ids instead of vLAPIC ids /2 [Andrew Cooper, Jan Beulich]
>> >> >> ---
>> >> >>  arch/arm/xen/enlighten.c | 10 ++++++++++
>> >> >>  arch/x86/xen/enlighten.c | 23 ++++++++++++++++++++++-
>> >> >>  include/xen/xen-ops.h    |  6 ++++++
>> >> >>  3 files changed, 38 insertions(+), 1 deletion(-)
>> >> >>
>> >> >> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
>> >> >> index 75cd734..fe32267 100644
>> >> >> --- a/arch/arm/xen/enlighten.c
>> >> >> +++ b/arch/arm/xen/enlighten.c
>> >> >> @@ -46,6 +46,10 @@ struct shared_info *HYPERVISOR_shared_info = (void *)&xen_dummy_shared_info;
>> >> >>  DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);
>> >> >>  static struct vcpu_info __percpu *xen_vcpu_info;
>> >> >>
>> >> >> +/* Linux <-> Xen vCPU id mapping */
>> >> >> +DEFINE_PER_CPU(uint32_t, xen_vcpu_id) = U32_MAX;
>> >> >> +EXPORT_PER_CPU_SYMBOL(xen_vcpu_id);
>> >> >> +
>> >> >>  /* These are unused until we support booting "pre-ballooned" */
>> >> >>  unsigned long xen_released_pages;
>> >> >>  struct xen_memory_region xen_extra_mem[XEN_EXTRA_MEM_MAX_REGIONS] __initdata;
>> >> >> @@ -179,6 +183,9 @@ static void xen_percpu_init(void)
>> >> >>  	pr_info("Xen: initializing cpu%d\n", cpu);
>> >> >>  	vcpup = per_cpu_ptr(xen_vcpu_info, cpu);
>> >> >>
>> >> >> +	/* Direct vCPU id mapping for ARM guests. */
>> >> >> +	per_cpu(xen_vcpu_id, cpu) = cpu;
>> >> >> +
>> >> >
>> >> > We did some internal testing on ARM64 with the latest Linux kernel
>> >> > (4.8-rc4) and noticed that this patch is breaking SMP support. Sorry
>> >> > for noticing the issue that late.
>> >> 
>> >> Sorry for the breakage :-(
>> >> 
>> >> >
>> >> > This function is called on the running CPU whilst some code (e.g
>> >> > init_control_block in drivers/xen/events/events_fifo.c) is executed
>> >> > whilst preparing the CPU on the boot CPU.
>> >> >
>> >> > So xen_vcpu_nr(cpu) will always return 0 in this case and
>> >> > init_control_block will fail to execute.
>> >> >
>> >> 
>> >> I see,
>> >> 
>> >> CPU_UP_PREPARE event happens before xen_starting_cpu() is called.
>> >> 
>> >> 
>> >> > I am not sure how to fix. I guess we could setup per_cpu(xen_vcpu_id,
>> >> > *) in xen_guest_init. Any opinions?
>> >> 
>> >> As we're not doing kexec on ARM we can fix the immediate issue. I don't
>> >> know much about ARM and unfortunatelly I don't have a setup to test but
>> >> it seems there is no early_per_cpu* infrastructure for ARM so we may fix
>> >> it with the following:
>> >> 
>> >> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
>> >> index 3d2cef6..f193414 100644
>> >> --- a/arch/arm/xen/enlighten.c
>> >> +++ b/arch/arm/xen/enlighten.c
>> >> @@ -170,9 +170,6 @@ static int xen_starting_cpu(unsigned int cpu)
>> >>         pr_info("Xen: initializing cpu%d\n", cpu);
>> >>         vcpup = per_cpu_ptr(xen_vcpu_info, cpu);
>> >>  
>> >> -       /* Direct vCPU id mapping for ARM guests. */
>> >> -       per_cpu(xen_vcpu_id, cpu) = cpu;
>> >> -
>> >>         info.mfn = virt_to_gfn(vcpup);
>> >>         info.offset = xen_offset_in_page(vcpup);
>> >>  
>> >> @@ -330,6 +327,7 @@ static int __init xen_guest_init(void)
>> >>  {
>> >>         struct xen_add_to_physmap xatp;
>> >>         struct shared_info *shared_info_page = NULL;
>> >> +       int cpu;
>> >>  
>> >>         if (!xen_domain())
>> >>                 return 0;
>> >> @@ -380,7 +378,8 @@ static int __init xen_guest_init(void)
>> >>                 return -ENOMEM;
>> >>  
>> >>         /* Direct vCPU id mapping for ARM guests. */
>> >> -       per_cpu(xen_vcpu_id, 0) = 0;
>> >> +       for_each_possible_cpu(cpu)
>> >> +               per_cpu(xen_vcpu_id, cpu) = cpu;
>> >>  
>> >>         xen_auto_xlat_grant_frames.count = gnttab_max_grant_frames();
>> >>         if (xen_xlate_map_ballooned_pages(&xen_auto_xlat_grant_frames.pfn,
>> >> 
>> >> (not tested, if we can't use for_each_possible_cpu() that early we'll
>> >> have to check against NR_CPUS instead).
>> >
>> > Kind of defeat the purpose of xen_vcpu_id, but I guess it should work.
>> >
>> >> But unfortunatelly we'll have to get back to this in future. Turns out
>> >> we need to know Xen's idea of vCPU id _before_ this vCPU starts
>> >> executing code.
>> >
>> > Why?
>> 
>> E.g. for FIFO event channels we currently call
>> evtchn_fifo_cpu_notification() for CPU_UP_PREPARE event (and do a
>> hypercall which requires us to know vcpu id) and this happens before the
>> secodary cpu starts. I'm not sure this can't be changed in future.
>> 
>> >
>> >> On x86 we used ACPI_ID from MADT. Is there anything like
>> >> it on ARM?
>> >
>> > MPIDR: 
>> >
>> > http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0388e/CIHEBGFG.html
>> >
>> 
>> Thanks, but doesn't it work the same way CPUID instruction works? In
>> that case we won't be able to use it to figure out other CPU's id.
>
> I don't know that much about cpuid, but the virtual MPIDR is constructed
> from the vcpu id right now:
>
>     v->arch.vmpidr = MPIDR_SMP | vcpuid_to_vaffinity(v->vcpu_id);
>
>     [...]
>
>     static inline register_t vcpuid_to_vaffinity(unsigned int vcpuid)
>     {
>         register_t vaff;
>
>         vaff = (vcpuid & 0x0f) << MPIDR_LEVEL_SHIFT(0);
>         vaff |= ((vcpuid >> 4) & MPIDR_LEVEL_MASK) << MPIDR_LEVEL_SHIFT(1);
>
>         return vaff;
>     }

This could work but only in case there is a way to get MPIDR for _other_
cpu (e.g. CPU0 needs to get MPIDR of CPU1 when CPU1 is not yet runnning)
or we'll have to change the machinery of how we bring up secondary CPUs
- e.g. CPUn starts, writes its id somewhere and 'hangs' waiting for CPU0
to set up event channels.

-- 
  Vitaly