Re: [PATCH 1/1] KVM: x86: fix MSR_IA32_TSC read for nested migration

[Sean Christopherson]

On Thu, Sep 17, 2020 at 02:07:23PM +0300, Maxim Levitsky wrote:
> MSR reads/writes should always access the L1 state, since the (nested)
> hypervisor should intercept all the msrs it wants to adjust, and these
> that it doesn't should be read by the guest as if the host had read it.
> 
> However IA32_TSC is an exception.Even when not intercepted, guest still

Missing a space after the period.

> reads the value + TSC offset.
> The write however does not take any TSC offset in the account.

s/in the/into

> This is documented in Intel's PRM and seems also to happen on AMD as well.

Ideally we'd get confirmation from AMD that this is the correct behavior.

> This creates a problem when userspace wants to read the IA32_TSC value and then
> write it. (e.g for migration)
> 
> In this case it reads L2 value but write is interpreted as an L1 value.

It _may_ read the L2 value, e.g. it's not going to read the L2 value if L1
is active.

> To fix this make the userspace initiated reads of IA32_TSC return L1 value
> as well.
> 
> Huge thanks to Dave Gilbert for helping me understand this very confusing
> semantic of MSR writes.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 17f4995e80a7e..d10d5c6add359 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2025,6 +2025,11 @@ u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
>  }
>  EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
>  
> +static u64 kvm_read_l2_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)

This is definitely not L2 specific.  I would vote to just omit the helper so
that we don't need to come up with a name that is correct across the board,
e.g. "raw" is also not quite correct.

An alternative would be to do:

	u64 tsc_offset = msr_info->host_initiated ? vcpu->arch.l1_tsc_offset :
						    vcpu->arch.tsc_offset;

	msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) + tsc_offset;

Which I kind of like because the behavioral difference is a bit more obvious.

> +{
> +	return vcpu->arch.tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> +}
> +
>  static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>  {
>  	vcpu->arch.l1_tsc_offset = offset;
> @@ -3220,7 +3225,19 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		msr_info->data = vcpu->arch.msr_ia32_power_ctl;
>  		break;
>  	case MSR_IA32_TSC:
> -		msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) + vcpu->arch.tsc_offset;
> +		/*
> +		 * Intel PRM states that MSR_IA32_TSC read adds the TSC offset
> +		 * even when not intercepted. AMD manual doesn't define this
> +		 * but appears to behave the same
> +		 *
> +		 * However when userspace wants to read this MSR, return its
> +		 * real L1 value so that its restore will be correct
> +		 *

Extra line is unnecessary.

> +		 */
> +		if (msr_info->host_initiated)
> +			msr_info->data = kvm_read_l1_tsc(vcpu, rdtsc());
> +		else
> +			msr_info->data = kvm_read_l2_tsc(vcpu, rdtsc());
>  		break;
>  	case MSR_MTRRcap:
>  	case 0x200 ... 0x2ff:
> -- 
> 2.26.2
>