Re: [PATCHv2] kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM

Alexander Graf
2015-02-04T15:24:46Z

On 03.02.15 06:44, David Gibson wrote:
> On POWER, storage caching is usually configured via the MMU - attributes
> such as cache-inhibited are stored in the TLB and the hashed page table.
>
> This makes correctly performing cache inhibited IO accesses awkward when
> the MMU is turned off (real mode). Some CPU models provide special
> registers to control the cache attributes of real mode load and stores but
> this is not at all consistent. This is a problem in particular for SLOF,
> the firmware used on KVM guests, which runs entirely in real mode, but
> which needs to do IO to load the kernel.
>
> To simplify this qemu implements two special hypercalls, H_LOGICAL_CI_LOAD
> and H_LOGICAL_CI_STORE which simulate a cache-inhibited load or store to
> a logical address (aka guest physical address). SLOF uses these for IO.
>
> However, because these are implemented within qemu, not the host kernel,
> these bypass any IO devices emulated within KVM itself. The simplest way
> to see this problem is to attempt to boot a KVM guest from a virtio-blk
> device with iothread / dataplane enabled. The iothread code relies on an
> in kernel implementation of the virtio queue notification, which is not
> triggered by the IO hcalls, and so the guest will stall in SLOF unable to
> load the guest OS.
>
> This patch addresses this by providing in-kernel implementations of the
> 2 hypercalls, which correctly scan the KVM IO bus. Any access to an
> address not handled by the KVM IO bus will cause a VM exit, hitting the
> qemu implementation as before.
>
> Note that a userspace change is also required, in order to enable these
> new hcall implementations with KVM_CAP_PPC_ENABLE_HCALL.
>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
> arch/powerpc/include/asm/kvm_book3s.h | 3 ++
> arch/powerpc/kvm/book3s.c | 76 +++++++++++++++++++++++++++++++++++
> arch/powerpc/kvm/book3s_hv.c | 12 ++++++
> arch/powerpc/kvm/book3s_pr_papr.c | 28 +++++++++++++
> 4 files changed, 119 insertions(+)
>
> v2:
> - Removed some debugging printk()s that were accidentally left in
> - Fix endianness; like all PAPR hypercalls, these should always act
> big-endian, even if the guest is little-endian (in practice this
> makes no difference, since the only user is SLOF, which is always
> big-endian)
>
> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> index 942c7b1..578e550 100644
> --- a/arch/powerpc/include/asm/kvm_book3s.h
> +++ b/arch/powerpc/include/asm/kvm_book3s.h
> @@ -292,6 +292,9 @@ static inline bool kvmppc_supports_magic_page(struct kvm_vcpu *vcpu)
> return !is_kvmppc_hv_enabled(vcpu->kvm);
> }
>
> +extern int kvmppc_h_logical_ci_load(struct kvm_vcpu *vcpu);
> +extern int kvmppc_h_logical_ci_store(struct kvm_vcpu *vcpu);
> +
> /* Magic register values loaded into r3 and r4 before the 'sc' assembly
> * instruction for the OSI hypercalls */
> #define OSI_SC_MAGIC_R3 0x113724FA
> diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> index 888bf46..7b51492 100644
> --- a/arch/powerpc/kvm/book3s.c
> +++ b/arch/powerpc/kvm/book3s.c
> @@ -820,6 +820,82 @@ void kvmppc_core_destroy_vm(struct kvm *kvm)
> #endif
> }
>
> +int kvmppc_h_logical_ci_load(struct kvm_vcpu *vcpu)
> +{
> + unsigned long size = kvmppc_get_gpr(vcpu, 4);
> + unsigned long addr = kvmppc_get_gpr(vcpu, 5);
> + u64 buf;
> + int ret;
> +
> + if (!is_power_of_2(size) || (size > sizeof(buf)))
> + return H_TOO_HARD;
> +
> + ret = kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, size, &buf);
> + if (ret != 0)
> + return H_TOO_HARD;
> +
> + switch (size) {
> + case 1:
> + kvmppc_set_gpr(vcpu, 4, *(u8 *)&buf);
> + break;
> +
> + case 2:
> + kvmppc_set_gpr(vcpu, 4, be16_to_cpu(*(u16 *)&buf));
> + break;
> +
> + case 4:
> + kvmppc_set_gpr(vcpu, 4, be32_to_cpu(*(u32 *)&buf));
> + break;
> +
> + case 8:
> + kvmppc_set_gpr(vcpu, 4, be64_to_cpu(*(u64 *)&buf));

Shouldn't these casts be __be types?

> + break;
> +
> + default:
> + BUG();
> + }
> +
> + return H_SUCCESS;
> +}
> +EXPORT_SYMBOL_GPL(kvmppc_h_logical_ci_load); /* For use by the kvm-pr module */

No need for the comment.

> +
> +int kvmppc_h_logical_ci_store(struct kvm_vcpu *vcpu)
> +{
> + unsigned long size = kvmppc_get_gpr(vcpu, 4);
> + unsigned long addr = kvmppc_get_gpr(vcpu, 5);
> + unsigned long val = kvmppc_get_gpr(vcpu, 6);
> + u64 buf;
> + int ret;
> +
> + switch (size) {
> + case 1:
> + *(u8 *)&buf = val;
> + break;
> +
> + case 2:
> + *(u16 *)&buf = cpu_to_be16(val);
> + break;
> +
> + case 4:
> + *(u32 *)&buf = cpu_to_be32(val);
> + break;
> +
> + case 8:
> + *(u64 *)&buf = cpu_to_be64(val);

Same thing about casts here.

Alex