linux-kernel - Re: [PATCH v4] KVM: kvmclock: Fix vCPUs

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Date:   Fri, 12 Mar 2021 19:18:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v4] KVM: kvmclock: Fix vCPUs > 64 can't be
 online/hotpluged

On 24/02/21 02:37, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> # lscpu
> Architecture:          x86_64
> CPU op-mode(s):        32-bit, 64-bit
> Byte Order:            Little Endian
> CPU(s):                88
> On-line CPU(s) list:   0-63
> Off-line CPU(s) list:  64-87
> 
> # cat /proc/cmdline
> BOOT_IMAGE=/vmlinuz-5.10.0-rc3-tlinux2-0050+ root=/dev/mapper/cl-root ro
> rd.lvm.lv=cl/root rhgb quiet console=ttyS0 LANG=en_US .UTF-8 no-kvmclock-vsyscall
> 
> # echo 1 > /sys/devices/system/cpu/cpu76/online
> -bash: echo: write error: Cannot allocate memory
> 
> The per-cpu vsyscall pvclock data pointer assigns either an element of the
> static array hv_clock_boot (#vCPU <= 64) or dynamically allocated memory
> hvclock_mem (vCPU > 64), the dynamically memory will not be allocated if
> kvmclock vsyscall is disabled, this can result in cpu hotpluged fails in
> kvmclock_setup_percpu() which returns -ENOMEM. It's broken for no-vsyscall
> and sometimes you end up with vsyscall disabled if the host does something
> strange. This patch fixes it by allocating this dynamically memory
> unconditionally even if vsyscall is disabled.
> 
> Fixes: 6a1cac56f4 ("x86/kvm: Use __bss_decrypted attribute in shared variables")
> Reported-by: Zelin Deng <zelin.deng@...ux.alibaba.com>
> Cc: Brijesh Singh <brijesh.singh@....com>
> Cc: stable@...r.kernel.org#v4.19-rc5+
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v3 -> v4:
>   * fix kernel test robot report WARNING
> v2 -> v3:
>   * allocate dynamically memory unconditionally
> v1 -> v2:
>   * add code comments
> 
>   arch/x86/kernel/kvmclock.c | 19 +++++++++----------
>   1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index aa59374..1fc0962 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -268,21 +268,20 @@ static void __init kvmclock_init_mem(void)
>   
>   static int __init kvm_setup_vsyscall_timeinfo(void)
>   {
> -#ifdef CONFIG_X86_64
> -	u8 flags;
> +	kvmclock_init_mem();
>   
> -	if (!per_cpu(hv_clock_per_cpu, 0) || !kvmclock_vsyscall)
> -		return 0;
> +#ifdef CONFIG_X86_64
> +	if (per_cpu(hv_clock_per_cpu, 0) && kvmclock_vsyscall) {
> +		u8 flags;
>   
> -	flags = pvclock_read_flags(&hv_clock_boot[0].pvti);
> -	if (!(flags & PVCLOCK_TSC_STABLE_BIT))
> -		return 0;
> +		flags = pvclock_read_flags(&hv_clock_boot[0].pvti);
> +		if (!(flags & PVCLOCK_TSC_STABLE_BIT))
> +			return 0;
>   
> -	kvm_clock.vdso_clock_mode = VDSO_CLOCKMODE_PVCLOCK;
> +		kvm_clock.vdso_clock_mode = VDSO_CLOCKMODE_PVCLOCK;
> +	}
>   #endif
>   
> -	kvmclock_init_mem();
> -
>   	return 0;
>   }
>   early_initcall(kvm_setup_vsyscall_timeinfo);
> 

Queued, thanks.

Paolo