linux-kernel - Re: [PATCH v3] x86/hyperv: Use per cpu initial stack for vtl context

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Date: Sun, 3 Mar 2024 23:08:17 -0800
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Wei Liu <wei.liu@...nel.org>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, decui@...rosoft.com,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	dwmw@...zon.co.uk, peterz@...radead.org,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	ssengar@...rosoft.com, mhklinux@...look.com
Subject: Re: [PATCH v3] x86/hyperv: Use per cpu initial stack for vtl context

On Mon, Mar 04, 2024 at 06:24:27AM +0000, Wei Liu wrote:
> On Sun, Mar 03, 2024 at 12:01:36AM -0800, Saurabh Sengar wrote:
> > Currently, the secondary CPUs in Hyper-V VTL context lack support for
> > parallel startup. Therefore, relying on the single initial_stack fetched
> > from the current task structure suffices for all vCPUs.
> > 
> > However, common initial_stack risks stack corruption when parallel startup
> > is enabled. In order to facilitate parallel startup, use the initial_stack
> > from the per CPU idle thread instead of the current task.
> > 
> > Fixes: 18415f33e2ac ("cpu/hotplug: Allow "parallel" bringup up to CPUHP_BP_KICK_AP_STATE")
> 
> I don't think this patch is buggy. Instead, it exposes an assumption in
> the VTL code. So this either should be dropped or point to the patch
> which introduces the assumption.
> 
> Let me know what you would prefer.

The VTL code will crash if this fix is not present post above mentioned patch:
18415f33e2ac ("cpu/hotplug: Allow "parallel" bringup up to CPUHP_BP_KICK_AP_STATE").
So I would prefer a fixes which added the assumption in VTL:

Fixes: 3be1bc2fe9d2 ("x86/hyperv: VTL support for Hyper-V")

Please let me know if you need V4 for it.

> 
> Thanks,
> Wei.
> 
> > Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> > Reviewed-by: Michael Kelley <mhklinux@...look.com>
> > ---
> > [V3]
> >  - Added the VTL code dependency on SMP to fix kernel build error
> >    when SMP is disabled.
> > 
> >  arch/x86/hyperv/hv_vtl.c | 19 +++++++++++++++----
> >  drivers/hv/Kconfig       |  1 +
> >  2 files changed, 16 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> > index 804b629ea49d..b4e233954d0f 100644
> > --- a/arch/x86/hyperv/hv_vtl.c
> > +++ b/arch/x86/hyperv/hv_vtl.c
> > @@ -12,6 +12,7 @@
> >  #include <asm/i8259.h>
> >  #include <asm/mshyperv.h>
> >  #include <asm/realmode.h>
> > +#include <../kernel/smpboot.h>
> >  
> >  extern struct boot_params boot_params;
> >  static struct real_mode_header hv_vtl_real_mode_header;
> > @@ -58,7 +59,7 @@ static void hv_vtl_ap_entry(void)
> >  	((secondary_startup_64_fn)secondary_startup_64)(&boot_params, &boot_params);
> >  }
> >  
> > -static int hv_vtl_bringup_vcpu(u32 target_vp_index, u64 eip_ignored)
> > +static int hv_vtl_bringup_vcpu(u32 target_vp_index, int cpu, u64 eip_ignored)
> >  {
> >  	u64 status;
> >  	int ret = 0;
> > @@ -72,7 +73,9 @@ static int hv_vtl_bringup_vcpu(u32 target_vp_index, u64 eip_ignored)
> >  	struct ldttss_desc *ldt;
> >  	struct desc_struct *gdt;
> >  
> > -	u64 rsp = current->thread.sp;
> > +	struct task_struct *idle = idle_thread_get(cpu);
> > +	u64 rsp = (unsigned long)idle->thread.sp;
> > +
> >  	u64 rip = (u64)&hv_vtl_ap_entry;
> >  
> >  	native_store_gdt(&gdt_ptr);
> > @@ -199,7 +202,15 @@ static int hv_vtl_apicid_to_vp_id(u32 apic_id)
> >  
> >  static int hv_vtl_wakeup_secondary_cpu(u32 apicid, unsigned long start_eip)
> >  {
> > -	int vp_id;
> > +	int vp_id, cpu;
> > +
> > +	/* Find the logical CPU for the APIC ID */
> > +	for_each_present_cpu(cpu) {
> > +		if (arch_match_cpu_phys_id(cpu, apicid))
> > +			break;
> > +	}
> > +	if (cpu >= nr_cpu_ids)
> > +		return -EINVAL;
> >  
> >  	pr_debug("Bringing up CPU with APIC ID %d in VTL2...\n", apicid);
> >  	vp_id = hv_vtl_apicid_to_vp_id(apicid);
> > @@ -213,7 +224,7 @@ static int hv_vtl_wakeup_secondary_cpu(u32 apicid, unsigned long start_eip)
> >  		return -EINVAL;
> >  	}
> >  
> > -	return hv_vtl_bringup_vcpu(vp_id, start_eip);
> > +	return hv_vtl_bringup_vcpu(vp_id, cpu, start_eip);
> >  }
> >  
> >  int __init hv_vtl_early_init(void)
> > diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> > index 00242107d62e..862c47b191af 100644
> > --- a/drivers/hv/Kconfig
> > +++ b/drivers/hv/Kconfig
> > @@ -16,6 +16,7 @@ config HYPERV
> >  config HYPERV_VTL_MODE
> >  	bool "Enable Linux to boot in VTL context"
> >  	depends on X86_64 && HYPERV
> > +	depends on SMP
> >  	default n
> >  	help
> >  	  Virtual Secure Mode (VSM) is a set of hypervisor capabilities and
> > -- 
> > 2.34.1
> >