[kvmtool PATCH v2 1/1] riscv: Fix the hart bit setting of AIA

[Andrew Jones]

On Wed, May 15, 2024 at 05:19:02PM GMT, Yong-Xuan Wang wrote:
> In AIA spec, each hart (or each hart within a group) has a unique hart
> number to locate the memory pages of interrupt files in the address
> space. The number of bits required to represent any hart number is equal
> to ceil(log2(hmax + 1)), where hmax is the largest hart number among
> groups.
> 
> However, if the largest hart number among groups is a power of 2, QEMU
> will pass an inaccurate hart-index-bit setting to Linux. For example, when
> the guest OS has 4 harts, only ceil(log2(3 + 1)) = 2 bits are sufficient
> to represent 4 harts, but we passes 3 to Linux. The code needs to be
> updated to ensure accurate hart-index-bit settings.
> 
> Additionally, a Linux patch[1] is necessary to correctly recover the hart
> index when the guest OS has only 1 hart, where the hart-index-bit is 0.
> 
> [1] https://lore.kernel.org/lkml/20240415064905.25184-1-yongxuan.wang@sifive.com/t/
> 
> Signed-off-by: Yong-Xuan Wang <yongxuan.wang at sifive.com>
> ---
> Changelog
> v2:
> - update commit message
> ---
>  riscv/aia.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/riscv/aia.c b/riscv/aia.c
> index fe9399a8ffc1..21d9704145d0 100644
> --- a/riscv/aia.c
> +++ b/riscv/aia.c
> @@ -164,7 +164,7 @@ static int aia__init(struct kvm *kvm)
>  	ret = ioctl(aia_fd, KVM_SET_DEVICE_ATTR, &aia_nr_sources_attr);
>  	if (ret)
>  		return ret;
> -	aia_hart_bits = fls_long(kvm->nrcpus);
> +	aia_hart_bits = fls_long(kvm->nrcpus - 1);
>  	ret = ioctl(aia_fd, KVM_SET_DEVICE_ATTR, &aia_hart_bits_attr);
>  	if (ret)
>  		return ret;
> -- 
> 2.17.1
>

Reviewed-by: Andrew Jones <ajones at ventanamicro.com>