Re: [PATCH] KVM: x86: Shove vp_bitmap handling down into sparse_set_to_vcpu_mask()

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> Move the vp_bitmap "allocation" that's need to handle mismatched vp_index
> values down into sparse_set_to_vcpu_mask() and drop __always_inline from
> said helper.  The vp_bitmap mess is a detail that's specific to the sparse
> translation and does not need to be exposed to the caller.
>
> The underlying motivation is to fudge around a compilation warning/error
> when CONFIG_KASAN_STACK=y, which is selected (and can't be unselected) by
> CONFIG_KASAN=y when compiling with gcc (clang/LLVM is a stack hog in some
> cases so it's opt-in for clang).  KASAN_STACK adds a redzone around every
> stack variable, which pushes the Hyper-V functions over the default limit
> of 1024.  With CONFIG_KVM_WERROR=y, this breaks the build.  Shuffling which
> function is charged with vp_bitmap gets all functions below the default
> limit.
>
> Regarding the __always_inline, prior to commit f21dd494506a ("KVM: x86:
> hyperv: optimize sparse VP set processing") the helper, then named
> hv_vcpu_in_sparse_set(), was a tiny bit of code that effectively boiled
> down to a handful of bit ops.  The __always_inline was understandable, if
> not justifiable.  Since the aforementioned change, sparse_set_to_vcpu_mask()
> is a chunky 350-450+ bytes of code without KASAN=y, and balloons to 1100+
> with KASAN=y.  In other words, it has no business being forcefully inlined.
>
> Reported-by: Ajay Garg <ajaygargnsit@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>
> Vitaly (and anyone with extensive KVM + Hyper-V knowledge), it would be
> really helpful to get better coverage in kvm-unit-tests.

I can't agree more. This *is* in my backlog but unfortunately I can't
give any forcast on when I'll get to it :-(

>  There's a smoke
> test for this in selftests, but it's not really all that interesting.  It
> took me over an hour and a half just to get a Linux guest to hit the
> relevant flows.  Most of that was due to QEMU 5.1 bugs (doesn't advertise
> HYPERCALL MSR by default)

This should be fixed already, right?

>  and Linux guest stupidity (silently disables
> itself if said MSR isn't available), but it was really annoying to have to
> go digging through QEMU to figure out how to even enable features that are
> extensive/critical enough to warrant their own tests.
>
> /wave to the clang folks for the pattern patch on the changelog ;-)
>
>  arch/x86/kvm/hyperv.c | 55 ++++++++++++++++++++++---------------------
>  1 file changed, 28 insertions(+), 27 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 4f15c0165c05..80018cfab5c7 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1710,31 +1710,36 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
>  		return kvm_hv_get_msr(vcpu, msr, pdata, host);
>  }
>  
> -static __always_inline unsigned long *sparse_set_to_vcpu_mask(
> -	struct kvm *kvm, u64 *sparse_banks, u64 valid_bank_mask,
> -	u64 *vp_bitmap, unsigned long *vcpu_bitmap)
> +static void sparse_set_to_vcpu_mask(struct kvm *kvm, u64 *sparse_banks,
> +				    u64 valid_bank_mask, unsigned long *vcpu_mask)
>  {
>  	struct kvm_hv *hv = to_kvm_hv(kvm);
> +	bool has_mismatch = atomic_read(&hv->num_mismatched_vp_indexes);
> +	u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
>  	struct kvm_vcpu *vcpu;
>  	int i, bank, sbank = 0;
> +	u64 *bitmap;
>  
> -	memset(vp_bitmap, 0,
> -	       KVM_HV_MAX_SPARSE_VCPU_SET_BITS * sizeof(*vp_bitmap));
> +	BUILD_BUG_ON(sizeof(vp_bitmap) >
> +		     sizeof(*vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS));
> +
> +	/* If vp_index == vcpu_idx for all vCPUs, fill vcpu_mask directly. */
> +	if (likely(!has_mismatch))
> +		bitmap = (u64 *)vcpu_mask;
> +
> +	memset(bitmap, 0, sizeof(vp_bitmap));

... but in the unlikely case has_mismatch == true 'bitmap' is still
uninitialized here, right? How doesn't it crash?

>  	for_each_set_bit(bank, (unsigned long *)&valid_bank_mask,
>  			 KVM_HV_MAX_SPARSE_VCPU_SET_BITS)
> -		vp_bitmap[bank] = sparse_banks[sbank++];
> +		bitmap[bank] = sparse_banks[sbank++];
>  
> -	if (likely(!atomic_read(&hv->num_mismatched_vp_indexes))) {
> -		/* for all vcpus vp_index == vcpu_idx */
> -		return (unsigned long *)vp_bitmap;
> -	}
> +	if (likely(!has_mismatch))
> +		return;
>  
> -	bitmap_zero(vcpu_bitmap, KVM_MAX_VCPUS);
> +	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		if (test_bit(kvm_hv_get_vpindex(vcpu), (unsigned long *)vp_bitmap))

'vp_bitmap' also doesn't seem to be assigned to anything, I'm really
confused :-(

Didn't you accidentally mix up 'vp_bitmap' and 'bitmap'?

> -			__set_bit(i, vcpu_bitmap);
> +			__set_bit(i, vcpu_mask);
>  	}
> -	return vcpu_bitmap;
>  }
>  
>  struct kvm_hv_hcall {
> @@ -1756,9 +1761,7 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
>  	struct kvm *kvm = vcpu->kvm;
>  	struct hv_tlb_flush_ex flush_ex;
>  	struct hv_tlb_flush flush;
> -	u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
> -	DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS);
> -	unsigned long *vcpu_mask;
> +	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
>  	u64 valid_bank_mask;
>  	u64 sparse_banks[64];
>  	int sparse_banks_len;
> @@ -1842,11 +1845,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
>  	if (all_cpus) {
>  		kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH_GUEST);
>  	} else {
> -		vcpu_mask = sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask,
> -						    vp_bitmap, vcpu_bitmap);
> +		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask);
>  
> -		kvm_make_vcpus_request_mask(kvm, KVM_REQ_TLB_FLUSH_GUEST,
> -					    vcpu_mask);
> +		kvm_make_vcpus_request_mask(kvm, KVM_REQ_TLB_FLUSH_GUEST, vcpu_mask);

We're not bound by 80-char limit anymore, are we? :-)

>  	}
>  
>  ret_success:
> @@ -1879,9 +1880,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
>  	struct kvm *kvm = vcpu->kvm;
>  	struct hv_send_ipi_ex send_ipi_ex;
>  	struct hv_send_ipi send_ipi;
> -	u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
> -	DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS);
> -	unsigned long *vcpu_mask;
> +	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
>  	unsigned long valid_bank_mask;
>  	u64 sparse_banks[64];
>  	int sparse_banks_len;
> @@ -1937,11 +1936,13 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
>  	if ((vector < HV_IPI_LOW_VECTOR) || (vector > HV_IPI_HIGH_VECTOR))
>  		return HV_STATUS_INVALID_HYPERCALL_INPUT;
>  
> -	vcpu_mask = all_cpus ? NULL :
> -		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask,
> -					vp_bitmap, vcpu_bitmap);
> +	if (all_cpus) {
> +		kvm_send_ipi_to_many(kvm, vector, NULL);
> +	} else {
> +		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask);
>  
> -	kvm_send_ipi_to_many(kvm, vector, vcpu_mask);
> +		kvm_send_ipi_to_many(kvm, vector, vcpu_mask);
> +	}
>  
>  ret_success:
>  	return HV_STATUS_SUCCESS;

-- 
Vitaly