pwndbg> x $fs
0x0 <fixed_percpu_data>:    Cannot access memory at address 0x0
pwndbg> x $fs:0
A syntax error in expression, near `:0'.
pwndbg> i r $fs
fs             0x0                 0
pwndbg> i r $fs:0
Invalid register `fs:0'
pwndbg>

Examine memory: x/FMT ADDRESS.
ADDRESS is an expression for the memory address to examine.
FMT is a repeat count followed by a format letter and a size letter.
Format letters are o(octal), x(hex), d(decimal), u(unsigned decimal),
  t(binary), f(float), a(address), i(instruction), c(char), s(string)
  and z(hex, zero padded on the left).
Size letters are b(byte), h(halfword), w(word), g(giant, 8 bytes).
The specified number of objects of the specified size are printed
according to the format.  If a negative number is specified, memory is
examined backward from the address.

Defaults for format and size letters are those previously used.
Default count is 1.  Default address is following last thing printed
with this command or "print".

pwndbg> p/x $rdi
$32 = 0xffff88803f7a1000
pwndbg> p/x $rax
$33 = 0x0
pwndbg> p/x $fs
$34 = 0x0
pwndbg> p/x $fs:0
A syntax error in expression, near `:0'.
pwndbg>