Re: [PATCH v3 2/4] hw/cxl: Use switch statements for read and write of cachemem registers

[Jonathan Cameron]

On Tue, 3 Oct 2023 16:42:37 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Sep 19, 2023 at 10:34:32AM +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>
> > ---
> >  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..7ef3ef2bd6 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 {
> > +            static_assert(sizeof(*cregs->cache_mem_registers) == 4);  
> 
> In C currently static_assert needs a message.

I switched to QEMU_BUILD_BUG_ON() which seems better
+ doesn't need a additional message.

> 
> In fact clang warns:
> ../hw/cxl/cxl-component-utils.c:75:67: error: '_Static_assert' with no 
> message is a C2x extension [-Werror,-Wc2x-extensions]
>             static_assert(sizeof(*cregs->cache_mem_registers) == 4);
> 
Thanks,

Jonathan

> 
> > +            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:
> > +        static_assert(sizeof(*cregs->cache_mem_regs_write_mask) == 4);
> > +        static_assert(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  
>