Subject: Re: [PATCH v5 14/69] KVM: arm64: nv: Support virtual EL2 exceptions

From: "Russell King (Oracle)" <[email-masked]>
Date: Tue, 18 Jan 2022 16:02:28 +0000

On Mon, Nov 29, 2021 at 08:00:55PM +0000, Marc Zyngier wrote:
> From: Jintack Lim <[email-masked]>
> 
> Support injecting exceptions and performing exception returns to and
> from virtual EL2.  This must be done entirely in software except when
> taking an exception from vEL0 to vEL2 when the virtual HCR_EL2.{E2H,TGE}
> == {1,1}  (a VHE guest hypervisor).
> 
> Signed-off-by: Jintack Lim <[email-masked]>
> Signed-off-by: Christoffer Dall <[email-masked]>
> [maz: switch to common exception injection framework]
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
...
> +void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
> +{
> +	u64 spsr, elr, mode;
> +	bool direct_eret;
> +
> +	/*
> +	 * Going through the whole put/load motions is a waste of time
> +	 * if this is a VHE guest hypervisor returning to its own
> +	 * userspace, or the hypervisor performing a local exception
> +	 * return. No need to save/restore registers, no need to
> +	 * switch S2 MMU. Just do the canonical ERET.
> +	 */
> +	spsr = vcpu_read_sys_reg(vcpu, SPSR_EL2);
> +	mode = spsr & (PSR_MODE_MASK | PSR_MODE32_BIT);
> +
> +	direct_eret  = (mode == PSR_MODE_EL0t &&
> +			vcpu_el2_e2h_is_set(vcpu) &&
> +			vcpu_el2_tge_is_set(vcpu));
> +	direct_eret |= (mode == PSR_MODE_EL2h || mode == PSR_MODE_EL2t);

There are excessive parens on the RHS of the above two.

> +static void kvm_inject_el2_exception(struct kvm_vcpu *vcpu, u64 esr_el2,
> +				     enum exception_type type)
> +{
> +	trace_kvm_inject_nested_exception(vcpu, esr_el2, type);
> +
> +	switch (type) {
> +	case except_type_sync:
> +		vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_ELx_SYNC;
> +		break;
> +	case except_type_irq:
> +		vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_ELx_IRQ;
> +		break;
> +	default:
> +		WARN_ONCE(1, "Unsupported EL2 exception injection %d\n", type);
> +	}
> +
> +	vcpu->arch.flags |= (KVM_ARM64_EXCEPT_AA64_EL2		|
> +			     KVM_ARM64_PENDING_EXCEPTION);

Ditto, and the "|" has too much white space before it with no good
reason.

> +/*
> + * Emulate taking an exception to EL2.
> + * See ARM ARM J8.1.2 AArch64.TakeException()
> + */
> +static int kvm_inject_nested(struct kvm_vcpu *vcpu, u64 esr_el2,
> +			     enum exception_type type)
> +{
> +	u64 pstate, mode;
> +	bool direct_inject;
> +
> +	if (!nested_virt_in_use(vcpu)) {
> +		kvm_err("Unexpected call to %s for the non-nesting configuration\n",
> +				__func__);

Too much indentation. I'm guessing this "unexpected" condition isn't
something that can be caused by a rogue guest? If it can, doesn't this
need to be rate limited?

> +		return -EINVAL;
> +	}
> +
> +	/*
> +	 * As for ERET, we can avoid doing too much on the injection path by
> +	 * checking that we either took the exception from a VHE host
> +	 * userspace or from vEL2. In these cases, there is no change in
> +	 * translation regime (or anything else), so let's do as little as
> +	 * possible.
> +	 */
> +	pstate = *vcpu_cpsr(vcpu);
> +	mode = pstate & (PSR_MODE_MASK | PSR_MODE32_BIT);
> +
> +	direct_inject  = (mode == PSR_MODE_EL0t &&
> +			  vcpu_el2_e2h_is_set(vcpu) &&
> +			  vcpu_el2_tge_is_set(vcpu));
> +	direct_inject |= (mode == PSR_MODE_EL2h || mode == PSR_MODE_EL2t);

Too many parens again...

> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
> index 0418399e0a20..4ef5e86efd8b 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
...
> @@ -149,7 +167,7 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
>  	new |= target_mode;
>  
>  	*vcpu_cpsr(vcpu) = new;
> -	__vcpu_write_spsr(vcpu, old);
> +	__vcpu_write_spsr(vcpu, target_mode, old);
>  }
>  
>  /*
> @@ -320,11 +338,22 @@ static void kvm_inject_exception(struct kvm_vcpu *vcpu)
>  		      KVM_ARM64_EXCEPT_AA64_EL1):
>  			enter_exception64(vcpu, PSR_MODE_EL1h, except_type_sync);
>  			break;
> +
> +		case (KVM_ARM64_EXCEPT_AA64_ELx_SYNC |
> +		      KVM_ARM64_EXCEPT_AA64_EL2):

Too many parens...

> +			enter_exception64(vcpu, PSR_MODE_EL2h, except_type_sync);
> +			break;
> +
> +		case (KVM_ARM64_EXCEPT_AA64_ELx_IRQ |
> +		      KVM_ARM64_EXCEPT_AA64_EL2):

Too many parens...

> diff --git a/arch/arm64/kvm/inject_fault.c b/arch/arm64/kvm/inject_fault.c
> index b47df73e98d7..5dcf3f8b08b8 100644
> --- a/arch/arm64/kvm/inject_fault.c
> +++ b/arch/arm64/kvm/inject_fault.c
> @@ -12,19 +12,58 @@
>  
>  #include <linux/kvm_host.h>
>  #include <asm/kvm_emulate.h>
> +#include <asm/kvm_nested.h>
>  #include <asm/esr.h>
>  
> +static void pend_sync_exception(struct kvm_vcpu *vcpu)
> +{
> +	vcpu->arch.flags |= (KVM_ARM64_EXCEPT_AA64_ELx_SYNC	|
> +			     KVM_ARM64_PENDING_EXCEPTION);

Too many parens...

> +
> +	/* If not nesting, EL1 is the only possible exception target */
> +	if (likely(!nested_virt_in_use(vcpu))) {
> +		vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_EL1;
> +		return;
> +	}
> +
> +	/*
> +	 * With NV, we need to pick between EL1 and EL2. Note that we
> +	 * never deal with a nesting exception here, hence never
> +	 * changing context, and the exception itself can be delayed
> +	 * until the next entry.
> +	 */
> +	switch(*vcpu_cpsr(vcpu) & PSR_MODE_MASK) {
> +	case PSR_MODE_EL2h:
> +	case PSR_MODE_EL2t:
> +		vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_EL2;
> +		break;
> +	case PSR_MODE_EL1h:
> +	case PSR_MODE_EL1t:
> +		vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_EL1;
> +		break;
> +	case PSR_MODE_EL0t:
> +		if (vcpu_el2_tge_is_set(vcpu) & HCR_TGE)
> +			vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_EL2;
> +		else
> +			vcpu->arch.flags |= KVM_ARM64_EXCEPT_AA64_EL1;
> +		break;
> +	default:
> +		BUG();

Is taking out the host really appropriate here? Is this something a
rogue guest could trigger?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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