Re: [PATCH 3/3] xhci-dbc: use brk_alloc()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.11.2025 13:39, Marek Marczykowski wrote:
> On Thu, Nov 13, 2025 at 12:10:16PM +0100, Jan Beulich wrote:
>> --- a/xen/drivers/char/xhci-dbc.c
>> +++ b/xen/drivers/char/xhci-dbc.c
>> @@ -27,6 +27,8 @@
>>  #include <xen/serial.h>
>>  #include <xen/timer.h>
>>  #include <xen/types.h>
>> +
>> +#include <asm/brk.h>
>>  #include <asm/fixmap.h>
>>  #include <asm/io.h>
>>  #include <asm/string.h>
>> @@ -1321,7 +1323,7 @@ static struct uart_driver dbc_uart_drive
>>  };
>>  
>>  /* Those are accessed via DMA. */
>> -struct dbc_dma_bufs {
>> +struct __aligned(PAGE_SIZE) dbc_dma_bufs {
>>      struct xhci_trb evt_trb[DBC_TRB_RING_CAP];
>>      struct xhci_trb out_trb[DBC_TRB_RING_CAP];
>>      struct xhci_trb in_trb[DBC_TRB_RING_CAP];
>> @@ -1335,8 +1337,7 @@ struct dbc_dma_bufs {
>>       * DMA-reachable by the USB controller.
>>       */
>>  };
>> -static struct dbc_dma_bufs __section(".bss.page_aligned") 
>> __aligned(PAGE_SIZE)
>> -    dbc_dma_bufs;
>> +DEFINE_BRK(xhci, sizeof(struct dbc_dma_bufs));
> 
> I think with this change (or rather with using brk_alloc() below), the
> structure wants to be padded up to the page size, to avoid putting
> anything else on the same page (see comment just outside of context
> above).

Are you sure? My understanding is that sizeof(xyz) is always evenly divisible by
alignof(xyz). Hence such padding doesn't need making explicit. (And yes, I did
see that comment while making the change.)

>> @@ -1413,24 +1414,33 @@ void __init xhci_dbc_uart_init(void)
>>  {
>>      struct dbc_uart *uart = &dbc_uart;
>>      struct dbc *dbc = &uart->dbc;
>> +    struct dbc_dma_bufs *dma_bufs;
>>  
>>      if ( !dbc->enable )
>>          return;
>>  
>> -    dbc->dbc_ctx = &dbc_dma_bufs.ctx;
>> -    dbc->dbc_erst = &dbc_dma_bufs.erst;
>> -    dbc->dbc_ering.trb = dbc_dma_bufs.evt_trb;
>> -    dbc->dbc_oring.trb = dbc_dma_bufs.out_trb;
>> -    dbc->dbc_iring.trb = dbc_dma_bufs.in_trb;
>> -    dbc->dbc_owork.buf = dbc_dma_bufs.out_wrk_buf;
>> -    dbc->dbc_iwork.buf = dbc_dma_bufs.in_wrk_buf;
>> -    dbc->dbc_str = dbc_dma_bufs.str_buf;
>> +    dma_bufs = brk_alloc(sizeof(*dma_bufs));
>> +    if ( !dma_bufs )
>> +    {
>> +        dbc->enable = false;
>> +        printk(XENLOG_ERR "XHCI: not enough BRK space available\n");
>> +        return;
>> +    }
>> +
>> +    dbc->dbc_ctx = &dma_bufs->ctx;
>> +    dbc->dbc_erst = &dma_bufs->erst;
>> +    dbc->dbc_ering.trb = dma_bufs->evt_trb;
>> +    dbc->dbc_oring.trb = dma_bufs->out_trb;
>> +    dbc->dbc_iring.trb = dma_bufs->in_trb;
>> +    dbc->dbc_owork.buf = dma_bufs->out_wrk_buf;
>> +    dbc->dbc_iwork.buf = dma_bufs->in_wrk_buf;
>> +    dbc->dbc_str = dma_bufs->str_buf;
>>  
>>      if ( dbc_open(dbc) )
>>      {
>>          iommu_add_extra_reserved_device_memory(
>> -                PFN_DOWN(virt_to_maddr(&dbc_dma_bufs)),
>> -                PFN_UP(sizeof(dbc_dma_bufs)),
>> +                PFN_DOWN(virt_to_maddr(dma_bufs)),
>> +                PFN_DOWN(sizeof(*dma_bufs)),
> 
> Is that really correct? But with padding (see earlier comment) it
> shouldn't really matter.

I think this is addressed by the reply further up as well.

Jan