linux-kernel - Re: [RFC PATCH v5 068/104] KVM: TDX: restore user ret MSRs

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Date:   Fri, 15 Apr 2022 16:06:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 068/104] KVM: TDX: restore user ret MSRs

On 3/4/22 20:49, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Several user ret MSRs are clobbered on TD exit.  Restore those values on
> TD exit and before returning to ring 3.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>   arch/x86/kvm/vmx/tdx.c | 33 +++++++++++++++++++++++++++++++++
>   1 file changed, 33 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 54be5be1a06c..c1366aac7d96 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -550,6 +550,28 @@ void tdx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>   	vcpu->kvm->vm_bugged = true;
>   }
>   
> +struct tdx_uret_msr {
> +	u32 msr;
> +	unsigned int slot;
> +	u64 defval;
> +};
> +
> +static struct tdx_uret_msr tdx_uret_msrs[] = {
> +	{.msr = MSR_SYSCALL_MASK,},
> +	{.msr = MSR_STAR,},
> +	{.msr = MSR_LSTAR,},
> +	{.msr = MSR_TSC_AUX,},
> +};
> +
> +static void tdx_user_return_update_cache(void)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(tdx_uret_msrs); i++)
> +		kvm_user_return_update_cache(tdx_uret_msrs[i].slot,
> +					     tdx_uret_msrs[i].defval);
> +}
> +
>   static void tdx_restore_host_xsave_state(struct kvm_vcpu *vcpu)
>   {
>   	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> @@ -589,6 +611,7 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu)
>   
>   	tdx_vcpu_enter_exit(vcpu, tdx);
>   
> +	tdx_user_return_update_cache();
>   	tdx_restore_host_xsave_state(vcpu);
>   	tdx->host_state_need_restore = true;
>   
> @@ -1371,6 +1394,16 @@ static int __init __tdx_hardware_setup(struct kvm_x86_ops *x86_ops)
>   	if (WARN_ON_ONCE(x86_ops->tlb_remote_flush))
>   		return -EIO;
>   
> +	for (i = 0; i < ARRAY_SIZE(tdx_uret_msrs); i++) {
> +		tdx_uret_msrs[i].slot = kvm_find_user_return_msr(tdx_uret_msrs[i].msr);
> +		if (tdx_uret_msrs[i].slot == -1) {
> +			/* If any MSR isn't supported, it is a KVM bug */
> +			pr_err("MSR %x isn't included by kvm_find_user_return_msr\n",
> +				tdx_uret_msrs[i].msr);
> +			return -EIO;
> +		}
> +	}
> +
>   	max_pkgs = topology_max_packages();
>   	tdx_mng_key_config_lock = kcalloc(max_pkgs, sizeof(*tdx_mng_key_config_lock),
>   				   GFP_KERNEL);

I wonder if you only need to do this if 
!this_cpu_ptr(user_return_msrs)->registered, but not a big deal.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>