linux-kernel - Re: [PATCH v2 5/5] KVM: nVMX: Allow VMREAD when Enlightened VMCS is in use

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Message-ID: <YdjKNcTcd2rFaA27@google.com>
Date:   Fri, 7 Jan 2022 23:18:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] KVM: nVMX: Allow VMREAD when Enlightened VMCS is
 in use

On Fri, Jan 07, 2022, Vitaly Kuznetsov wrote:
> Hyper-V TLFS explicitly forbids VMREAD and VMWRITE instructions when
> Enlightened VMCS interface is in use:
> 
> "Any VMREAD or VMWRITE instructions while an enlightened VMCS is
> active is unsupported and can result in unexpected behavior.""
> 
> Windows 11 + WSL2 seems to ignore this, attempts to VMREAD VMCS field
> 0x4404 ("VM-exit interruption information") are observed. Failing
> these attempts with nested_vmx_failInvalid() makes such guests
> unbootable.
> 
> Microsoft confirms this is a Hyper-V bug and claims that it'll get fixed
> eventually but for the time being we need a workaround. (Temporary) allow
> VMREAD to get data from the currently loaded Enlightened VMCS.
> 
> Note: VMWRITE instructions remain forbidden, it is not clear how to
> handle them properly and hopefully won't ever be needed.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

> +		/*
> +		 * Hyper-V TLFS (as of 6.0b) explicitly states, that while an
> +		 * enlightened VMCS is active VMREAD/VMWRITE instructions are
> +		 * unsupported. Unfortunately, certain versions of Windows 11
> +		 * don't comply with this requirement which is not enforced in
> +		 * genuine Hyper-V so KVM has implement a workaround allowing to

Nit, missing a "to".  But rather say what KVM "has" to do, maybe this?

		 * genuine Hyper-V.  Allow VMREAD from an enlightened VMCS as a
		 * workaround, as misbehaving guests will panic on VM-Fail.

> +		 * read from enlightened VMCS with VMREAD.
> +		 * Note, enlightened VMCS is incompatible with shadow VMCS so
> +		 * all VMREADs from L2 should go to L1.
> +		 */
> +		if (WARN_ON_ONCE(is_guest_mode(vcpu)))
> +			return nested_vmx_failInvalid(vcpu);
>  
> -	/* Read the field, zero-extended to a u64 value */
> -	value = vmcs12_read_any(vmcs12, field, offset);
> +		offset = evmcs_field_offset(field, NULL);
> +		if (offset < 0)
> +			return nested_vmx_fail(vcpu, VMXERR_UNSUPPORTED_VMCS_COMPONENT);
> +
> +		/* Read the field, zero-extended to a u64 value */
> +		value = evmcs_read_any(vmx->nested.hv_evmcs, field, offset);
> +	}
>  
>  	/*
>  	 * Now copy part of this value to register or memory, as requested.
> -- 
> 2.33.1
>