Re: [PATCH v4 2/4] hw/cxl: Use switch statements for read and write of cachemem registers

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 12, 2023 at 03:05:12PM +0100, Jonathan Cameron wrote:
> Establishing that only register accesses of size 4 and 8 can occur
> using these functions requires looking at their callers. Make it
> easier to see that by using switch statements.
> Assertions are used to enforce that the register storage is of the
> matching size, allowing fixed values to be used for divisors of
> the array indices.
> 
> Suggested-by: Michael Tokarev <mjt@tls.msk.ru>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Reviewed-by: Fan Ni <fan.ni@samsung.com>

Fails with clang:

../hw/cxl/cxl-component-utils.c:130:9: error: expected expression
        QEMU_BUILD_BUG_ON(sizeof(*cregs->cache_mem_regs_write_mask) != 4);
        ^
../include/qemu/compiler.h:76:30: note: expanded from macro 'QEMU_BUILD_BUG_ON'
#define QEMU_BUILD_BUG_ON(x) QEMU_BUILD_BUG_MSG(x, "not expecting: " #x)
                             ^
../include/qemu/compiler.h:74:36: note: expanded from macro 'QEMU_BUILD_BUG_MSG'
#define QEMU_BUILD_BUG_MSG(x, msg) _Static_assert(!(x), msg)
                                   ^
1 error generated.

See e.g.

https://gitlab.com/mstredhat/qemu/-/jobs/5320879531



> ---
> v4: Use QEMU_BUILD_BUG_ON() instead of static_assert with missing
>     error message.
> 
>  hw/cxl/cxl-component-utils.c | 65 +++++++++++++++++++++++-------------
>  1 file changed, 42 insertions(+), 23 deletions(-)
> 
> diff --git a/hw/cxl/cxl-component-utils.c b/hw/cxl/cxl-component-utils.c
> index f3bbf0fd13..d1997a52e6 100644
> --- a/hw/cxl/cxl-component-utils.c
> +++ b/hw/cxl/cxl-component-utils.c
> @@ -67,16 +67,24 @@ static uint64_t cxl_cache_mem_read_reg(void *opaque, hwaddr offset,
>      CXLComponentState *cxl_cstate = opaque;
>      ComponentRegisters *cregs = &cxl_cstate->crb;
>  
> -    if (size == 8) {
> +    switch (size) {
> +    case 4:
> +        if (cregs->special_ops && cregs->special_ops->read) {
> +            return cregs->special_ops->read(cxl_cstate, offset, 4);
> +        } else {
> +            QEMU_BUILD_BUG_ON(sizeof(*cregs->cache_mem_registers) != 4);
> +            return cregs->cache_mem_registers[offset / 4];
> +        }
> +    case 8:
>          qemu_log_mask(LOG_UNIMP,
>                        "CXL 8 byte cache mem registers not implemented\n");
>          return 0;
> -    }
> -
> -    if (cregs->special_ops && cregs->special_ops->read) {
> -        return cregs->special_ops->read(cxl_cstate, offset, size);
> -    } else {
> -        return cregs->cache_mem_registers[offset / sizeof(*cregs->cache_mem_registers)];
> +    default:
> +        /*
> +         * In line with specifiction limitaions on access sizes, this
> +         * routine is not called with other sizes.
> +         */
> +        g_assert_not_reached();
>      }
>  }
>  
> @@ -117,25 +125,36 @@ static void cxl_cache_mem_write_reg(void *opaque, hwaddr offset, uint64_t value,
>      ComponentRegisters *cregs = &cxl_cstate->crb;
>      uint32_t mask;
>  
> -    if (size == 8) {
> +    switch (size) {
> +    case 4:
> +        QEMU_BUILD_BUG_ON(sizeof(*cregs->cache_mem_regs_write_mask) != 4);
> +        QEMU_BUILD_BUG_ON(sizeof(*cregs->cache_mem_registers) != 4);
> +        mask = cregs->cache_mem_regs_write_mask[offset / 4];
> +        value &= mask;
> +        /* RO bits should remain constant. Done by reading existing value */
> +        value |= ~mask & cregs->cache_mem_registers[offset / 4];
> +        if (cregs->special_ops && cregs->special_ops->write) {
> +            cregs->special_ops->write(cxl_cstate, offset, value, size);
> +            return;
> +        }
> +
> +        if (offset >= A_CXL_HDM_DECODER_CAPABILITY &&
> +            offset <= A_CXL_HDM_DECODER3_TARGET_LIST_HI) {
> +            dumb_hdm_handler(cxl_cstate, offset, value);
> +        } else {
> +            cregs->cache_mem_registers[offset / 4] = value;
> +        }
> +        return;
> +    case 8:
>          qemu_log_mask(LOG_UNIMP,
>                        "CXL 8 byte cache mem registers not implemented\n");
>          return;
> -    }
> -    mask = cregs->cache_mem_regs_write_mask[offset / sizeof(*cregs->cache_mem_regs_write_mask)];
> -    value &= mask;
> -    /* RO bits should remain constant. Done by reading existing value */
> -    value |= ~mask & cregs->cache_mem_registers[offset / sizeof(*cregs->cache_mem_registers)];
> -    if (cregs->special_ops && cregs->special_ops->write) {
> -        cregs->special_ops->write(cxl_cstate, offset, value, size);
> -        return;
> -    }
> -
> -    if (offset >= A_CXL_HDM_DECODER_CAPABILITY &&
> -        offset <= A_CXL_HDM_DECODER3_TARGET_LIST_HI) {
> -        dumb_hdm_handler(cxl_cstate, offset, value);
> -    } else {
> -        cregs->cache_mem_registers[offset / sizeof(*cregs->cache_mem_registers)] = value;
> +    default:
> +        /*
> +         * In line with specifiction limitaions on access sizes, this
> +         * routine is not called with other sizes.
> +         */
> +        g_assert_not_reached();
>      }
>  }
>  
> -- 
> 2.39.2